Space Weather Studies

A. INTRODUCTION

Space Weather Studies investigates key scientific questions concerning electron density

enhancements in the lower ionosphere and middle atmosphere which are produced by relativistic electron

precipitation from the Earth’s outer radiation belts. The ionospheric and mesospheric signatures of both

steady and burst electron precipitation are to be measured at regular intervals via their effects on very

low frequency (VLF) subionospheric signals transmitted from Maine (NAA), Washington (NLK), and

Hawaii (NPM) and received at Fort Yukon and other ground stations in Alaska. (Figure 1). Our proposed

study will make use of relativistic electron data from the SAMPEX, UARS, and POLAR spacecraft. This

work is in association with Dr J. B. Blake, a Co-Investigator on SAMPEX associated

with the Proton / Electron Telescope (PET) instrument, Dr. D. L. Chenette, a co-investigator on UARS

associated with the High-Energy Particle Spectrometer (HEPS), and Dr. M. Walt, a Co-investigator on

POLAR associated with the CEPPAD/SEPS instrument.

Fig. 1. Subionospheric propagation paths from VLF transmitters in Washington (NLK), Maine (NAA), and Hawaii (NPM) to Fort Yukon (FY). The paths are superimposed on a world map showing the global average distribution of >400 kev electron precipitation as measured on the lowaltitude SAMPEX satellite [(Baker et al., 1995]. Significent portions of the propagation paths lie within the precipitation region and these paths are ideally situated to continuously monitor the ionospheric and atmospheric effects of both steady and burst electron precipitation.

The study has two main goals

1) to continuously measure electron density enhancements in the ionosphere and middle atmosphere

produced by precipitating relativistic electron fluxes in the subauroral region of the North American

Continent by means of a ground-based VLF remote diagnostic technique

2) to compare these enhancements with those predicted to occur due to precipitated relativistic electron

fluxes observed on various NASA spacecraft in order to provide a calibration for the ground measurements.

The long term goal is to provide within 5 years an operating VLF diagnostic system which can provide a

global picture of the ionospheric and atmospheric effects of relativistic electron precipitation as a function of time.

This work is directly relevant to the objectives of the National Space Weather Program, and in particular

addresses:

1) the coupling between the solar wind and the magnetosphere

2) improved global ionospheric specification and forecast and the evolution of ionospheric irregularities, with

particular emphasis on those processes affecting communication and navigation systems.

It also addresses the recommendations of the National Academy of Sciences report, Solar Influences on global

Change, to "monitor continuously the energetic particle inputs to the Earths’ atmosphere" and to "understand,

through in in situ measurements, the relationship of space-based measurements to the energy spectrum and

fluxes of both solar and galactic energetic particles reaching different altitudes in the Earths’ atmosphere."

The proposed work is highly leveraged since the instruments necessary to perform the ground-based

VLF measurements have already been developed under the sponsorship of the HAARP program.

In carrying out our study we plan to use spacecraft data already in the public domain and will

not require data in the validation phase.

The fluxes of relativistic (>1MeV) electrons in the outer magnetosphere at subauroral latitudes (4.5 <

L < 7) have been known to undergo pronounced increases and decreases [e.g., Baker et al., 1986;

Nagai, 1988]. With the POLAR spacecraft, unique high resolution and high sensitivity measurements of

relativistic electrons are currently being collected at high altitudes [Blake et al., 1995] while the associated

precipitation of these energetic electrons into the atmosphere is documented with data from the SAMPEX

[Baker et al., 1993a; 1994] and UARS missions [Gaines et al., 1994]. At geosynchronous orbit, where

large numbers of high-energy electrons are present and exhibit great variability in flux [e.g., Baker et

al., 1979], the highly penetrating relativistic particles have deleterious effects on spacecraft subsystems

[e.g., Reagan et al., 1983; Baker, 1985; Gussenhoven et al., 1987]. When they precipitate into the lower

ionosphere and mesosphere, these highly energetic electrons penetrate to altitudes as low as 40-60 km

with an energy flux which is 3-4 orders of magnitude greater than the galactic cosmic ray or solar EUV

deposition [Reagan, 1977; Baker et al., 1987; 1993b; Gaines et al., 1994], and may well affect the neutral

and ion chemistry of the middle atmosphere [Spear et al., 1984; Rusch et al., 1981; Solomon et al., 1981;

Callis et al., 1991]. The rate of energy deposition into the upper atmosphere during such enhancements

may be as high as ~ 1019 - 1020 ergs/day [Imhof and Gaines, 1993]. Global distributions of relativistic

electron precipitation regions measured on the SAMPEX satellite are shown in Figure 2.

VLF sounding (i.e., the measurement of the amplitude and phase of subionospheric signals) is a

sensitive tool for the measurement of ionospheric and atmospheric conductivity (i.e., electron density

and temperature), especially at altitudes below 90 km [e.g., Sechrist, 1974], and some of the early work

on relativistic electron precipitation events relied on subionospheric VLF measurements [e.g., Thorne

and Larsen, 1976]. In recent years, the VLF remote sensing method has been extensively utilized to study

a variety of lower ionospheric and atmospheric disturbances, including those associated with lightning

discharges [e.g., Inan et al., 1993; Burgess and Inan, 1993], heating by HF [Barr et al., 1985; Bell

et al., 1993] and VLF waves [Rodriguez and Inan, 1994], the auroral electrojet [Kikuchi and Evans,

1983; Cummer et al., 1996], and high-energy auroral particle precipitation [Cummer et al., 1997]. New

computer-based models of VLF propagation and scattering have also been recently developed [Poulsen et

al., 1990; 1993; Smith and Cotton, 1990] and the VLF method can now be quantitatively used to interpret

ionospheric signatures of relativistic electron precipitation in terms of their spatial extent and the altitude

profiles of ionization. Further discussion concerning model interpretations is provided in section C.2.

Fig. 2. Daily averaged global images of > 400 keV electron precipitation as measured on the low altitude SAMPEX satellite [Baker et al., 1995]. The three insets show the data from different days, as indicated (e.g., 93161 as day 161 of 1993), with the images constructed by averaging between 16-orbits per day. The sequence illustrates relativistic electron precipitation fluxes over a 33 day period. The data from day 93179 shows a typical ‘quiet’ period. The persistent intense precipitation over South America is due to the South Atlantic Anomaly (SAA). The lower right inset shows the configuration of the proposed experiment, illustrating that a VLF receiver at Fort Yukon would be uniquely positioned to study this phenomenon.

Fort Yukon is an excellent location for the VLF diagnostic for several important reasons.

First of all, the NLK-Fort Yukon (FY) path lies entirely within the nominal precipitation zone, which

minimizes the possible effects of other ionospheric disturbances. Second, the three propagation paths

provide coverage over the entire North American sector. Thus local time variations in the magnitude

and spatial (i.e., L-shell) extent of relativitic electron precipitation can be directly determined with

observations at regular intervals over the course of a 24-hour period.

In view of these considerations, we use the VLF method to quantitatively determine ionospheric

and atmospheric signatures of relativistic electron precipitation by using our robust models of

VLF propagation in the Earth-ionosphere waveguide [Poulsen et al., 1993; Cotton and Smith, 1991]. The

predicted signature of a typical enhancement event as would be observed with the NLK signal received

at Fort Yukon is shown in Figure 3. We note that the signal amplitude and phase changes by ~7 dB and

~ 70. respectively in response to a relativistic electron precipitation event. Changes of this magnitude

are readily detected in the data.

The observations of the NAA, NLK, and NPM signals at Fort Yukon will provide the core dataset for

the first year of our program on the basis of which the scientific questions discussed in the next section

will be investigated. The unique disposition of the three propagation paths with respect to the ionospheric

regions affected by relativistic electron precipitation is illustrated in Figures 1 and 2. In view of the relatively

low data rates involved in the proposed VLF operations all of the VLF phase and amplitude data acquired

at Fort Yukon will be brought back in near-real-time and will be made available to the scientific community

at large over the World Wide Web.

In the second and third year of our proposed program VLF data will also be acquired at three additional

ground stations on the west coast of Alaska. These measurements will allow the determination of electron

density enhancements with high spatial resolution in both altitude and horizontal distance.

Although for reasons of cost our proposal focuses solely on the North American continent, the number

of receiving sites could be readily expanded in the future to provide a truely global picture of the

ionospheric and atmospheric effects of relativistic particle precipitation as a function of time. Thus we

believe that the proposed system of remote monitoring of relativistic electron precipitation has great

promise for improving operational space weather capabilities within the next five years.

The proposed program synergistically benefits from existing Stanford research efforts, in particular a

program of VLF D-region diagnostics undertaken as part of the Air Force HF Active Auroral Research

Program (HAARP). As part of HAARP, highly sensitive and versatile (fully digital) VLF narrowband

amplitude/phase receivers have been built and will be installed and operated at four different sites in

Alaska. For HAARP, the VLF data will be analyzed largely for the purpose of extracting the electron

density and collision frequency profiles in the ionospheric spot heated by theHAARPHFheater. However,

the data acquired is stored on a continuous basis at full resolution and is thus directly available for the

purposes of the proposed program. It should be noted that analysis of this data to extract information

on the ionospheric/mesospheric signatures of relativistic electron precipitation requires a major effort as

described in later sections, which is not funded under the HAARP project.

The scientific background and outstanding questions that motivate the proposed study are provided in

the next two sections followed by a description of the proposed program. Collaborative investigations

and research personnel are described respectively in sections D and E.

Fig. 3. Predicted VLF signature of a typical relativistic electron enhancement event. a) VLF signal path geometry from the NAA transmitter in Maine, the NLK transmitter in Washington and the NPM transmitter in Hawaii to a receiver at Fort Yukon, with respect to the disturbed ionospheric region of relativistic electron precipitation (shaded). b) crossectional view of the earth ionospherewaveguide depicting the perturbation of the mode structure of the subionospheric signal due to relativistic electron precipitation. c) ionizational profiles resulting from relativistic electron precipitation fluxes corresponding to the different levels (1,2,3) as shown in (d), under relatively dense ambient D-region conditions, typical of the auroral and subauroral regions. d) a typical relativistic electron precipitation enhancement, shown here rising and falling within 5 days. In most cases, event durations are as much as 10-15 days. The flux levels and energy spectra of the precipitation was taken to be as given by Gaines et al. [1995], based on measurements on the UARS satellite. e) the predicted NLK signal amplitude variation as observed at Fort Yukon. f) the predicted NLK signal phase variation at Fort Yukon.

B. SCIENTIFIC BACKGROUND AND QUESTIONS

Relativistic Electron Precipitation

The Earth’s outer magnetosphere is often populated to a surprising degree by relativistic electrons

[Paulikas and Blake, 1979; Baker et al., 1979]. The origin of the multi-MeV electrons observed at

geosynchronous orbit and the source of the pronounced fluctuations in their intensity are not known

[Baker et al., 1987], although they are generally correlated with the onset of substorm activity [Bailey,

1968; Thorne and Larsen, 1976; Nagai, 1988]. Intensity enhancements occur with relatively regular

27-day periodicity and are well associated with solar wind stream structures [Baker et al., 1986].

The phenomena occurs over a variety of time scales; the highly relativistic component (3-10 MeV)

exhibits intensity enhancements which typically rise on a 2- to 3-day time scale and decay on a 3- to

4-day scale [Baker et al., 1986], while the so-called relativistic electron precipitation (REP) events, rise

to a maximum in 20-30 minutes and decay over 1-5 hours [Rosenberg et al., 1972]. Further, extreme

decreases in particle flux at geosynchronous orbit lasting 10-30 minutes that occur during substorm

growth phases [Baker and McPherron, 1990] and drifting holes in >300 keV electron data lasting 1-7

minutes and following substorm onset [Sergeev et al., 1992] have recently been observed.

While much of the systematic data on relativistic electron enhancements was from geosynchronous

orbit [e.g., Baker et al., 1979; 1986], the associated precipitation of these energetic particles into the

upper atmosphere was measured from the ground using HF [Bailey, 1968] and VLF [Thorne and Larsen,

1976] techniques, riometer and VHF scatter [Rosenberg et al., 1972] methods, from balloon-based x-ray

detectors [e.g., Parks et al., 1979], and from rocket-based platforms [e.g., Herraro et al., 1991]. Recent

data from the SAMPEX [Baker et al., 1993a; 1994], UARS [Gaines et al., 1994] and POLAR [Blake et al

1995] missions now facilitate systematic measurements of relativistic electron precipitation. In addition

to the relatively steady enhancements which rise and fall over many days, intense short duration (0.1-10

sec) bursts (narrow spikes) of >1 MeV electrons have been observed [Imhof et al., 1991; 1992], near the

midnight trapping boundary (4< L <6). However, it is not yet clear whether these ‘burst’ events are

temporal or spatial in character [Imhof et al., 1992; Blake et al., 1993].

Understanding the circumstances (magnitude and spatial distribution) of this precipitation is also

important from the standpoint of the associated effects in the lower ionosphere and mesosphere. The

possible role of the precipitating high energy populations in coupling the magnetosphere to the mesosphere

and in affecting the chemistry of the middle atmosphere is of great potential significance [Baker et al.,

1987; 1993b]. There is evidence which suggests that relativistic electron precipitation events may

induce significant (10 - 20%) ozone depletions at high latitudes [Spear et al., 1984; Callis et al., 1991].

However, results of one case study has highlighted the need for simultaneous measurements of associated

secondary ionization profiles before making definitive conclusions [Aikin, 1992]. If such a connection

exists, precipitation associated with relativistic electron enhancements could impose a modulating effect

(27-day and 11-year cycles of solar wind and magnetospheric variability) on the lower D-region ionization

and, possibly, on the upper level ozone chemistry [Baker et al., 1987]. We note, however, that the 27-day

periodicity is itself solar-cycle dependent.

Questions:

What are the ionospheric and atmospheric signatures of relativistic electron intensity

enhancements? Where does such precipitation occur? Is there a 27-day periodicity in the precipitating

component? Are the intensifications of the precipitating component related to substorm onset in

the same way as the enhancements at geosynchronous altitude? What is the local time distribution

of relativistic electron precipitation? What are the ionospheric and mesospheric signatures of short

duration (< 10-s) relativistic precipitation bursts observed on low altitude satellites? What is the

relationship of relativistic electron precipitation to the trapping boundary?

Approach:

As seen from Figure 1, the propagation paths between the VLF transmitters NAA, NLK,

and NPM to a receiver located at Fort Yukon are well suited for the measurement of ionospheric effects

of relativistic electron enhancements. Systematic data on the amplitude and phase of the VLF signal

will be interpreted in the context of theoretical models of VLF propagation and scattering to determine

the altitude profile of the electron density enhancement produced by the precipitating electrons (i.e., the

ionization profile; see Figure 3) and the spatial (i.e., L-shell) extent of the affected ionospheric regions.

Measurements over the course of a 24-hour period will allow the determination of local time variations.

With synoptic operations (e.g., 1-minute out of every 15-minutes), the longer term enhancements (i.e.,

days) as well as bursty precipitation (i.e., 0.1-10 sec) can be measured. If enhancements observed

on spacecraft as short duration events are spatially confined to the trapping boundary they would

not appear bursty in ground-based VLF data. If, on the other hand, they are bursty in nature, they

would appear to have rapid onsets and relatively slow decays corresponding to recovery of secondary

ionization, which may be interpreted in the context of D-region chemistry models to ascertain the

energy spectrum of the precipitation [Glukhov et al., 1992].

Questions:

What are the effects in the lower ionosphere of relativistic electron precipitation? What

are the profiles of enhanced electron density in the mesosphere and D-region during these events?

Under what conditions can relativistic electrons cause a significant reduction in atmospheric ozone at

high altitudes?

Approach:

The fact that the NLK-FY propagation path lies entirely within the nominal precipitation

zone allows for easy isolation of the effects of relativistic electron intensity enhancements and

quantitative interpretation of signal amplitude and phase variations in terms of the altitude profile of

ionization using available VLF propagation models [Poulsen et al., 1993]. As can be seen from Figure

3, altitude profiles of ionization expected to be produced by relativistic electron enhancements can

cause large and distinctly identifiable changes in the signal amplitude and phase. We can thus expect

to determine the resultant ionospheric density profiles by interpreting data on the amplitude and phase

of the VLF signal in the light of VLF propagation models. The accuracy of such a determination is

enhanced because the L-shell extent of the enhancements is now well known from SAMPEX. Once

the altitude profiles of the associated ionization enhancements are determined, the resultant effects on

the production and loss of stratospheric odd nitrogen and ozone can be evaluated [Callis et al., 1991;

Arkin, 1992].

C. THE RESEARCH

The scientific questions will be addressed by means of the VLF remote diagnostic technique.

First we note that the phase and amplitude measurement of each VLF signal provides two parameters

with which to characterize the altitude profile of the ionization enhancement along the propagation

path. For a single signal we could then use the phase and amplitude data to derive, for example, a two

parameter altitude profile of the average ionization enhancement along the propagation path. For two

different signals traversing the same path, the phase and amplitude measurements could be used to derive

for example either a four parameter altitude profile of the average ionization enhancement along the

propagation path or alternatively a three parameter altitude profile and a measure of the latitude at which

the maximum ionization enhancement is produced. Clearly more details of the ionization enhancements

can be determined if multiple VLF signals are monitored which traverse essentially the same perturbed

path. Since the NPM and NLK paths are relatively close within the precipitation belt (Fig 3a), the

perturbed ionospheres should be very similar if the intensity of the relativistic electron precipitation

is not a strong function of longitude. An example of the longitudinal dependence of the SAMPEX

precipitated fluxes is shown in Figure 4. It shows very little longitudinal dependence. We will examine

additional SAMPEX data to verify this result.

Fig. 4. SAMPEX > 400 kev fluxes as a function of L shell for two consecutive orbits over the Alaskan sector. The ground tracks of the orbits are separated by ~1500 km at the latitude of Fort Yukon.

As shown in Figure 1, major segments of the propagation paths to be monitored lie within the nominal

region of precipitation of the relativistic electrons. Thus the amplitude/phase variations can be very

directly interpreted in terms of the disturbed ionospheric and atmospheric electron density profile in

the precipitation region. An example of the phase and emplitude changes that could be produced by

precipitated relativistic fluxes is shown in Figure 3. For this example typical particle data was used as

input to the Stanford VLF Remote Diagnostics Code in order to predict the VLF signal amplitude and

phase changes at Fort Yukon that would be produced by these fluxes.

One of the goals of the proposed program is to demonstrate that the VLF remote diagnostic technique

can be used on a global scale to monitor relativistic electron precipitation. For this demonstration it

is convenient to initially use spacecraft data that is already in the public domain. In the second and

third year of our program however we will also use spacecraft data which is acquired in those years and

subsequently entered into the public domain.

During the first year of our proposed program we will focus on VLF data from Fort Yukon acquired

during 1992, 1993, and 1997, since SAMPEX data is readily available for these periods. We will use the

NPMand NLK data to determine a four parameter average electron density profile across the precipitation

region for a number of case studies for which SAMPEX or UARS particle data is available near the NPM

and NLK propagation paths. We will then use the satellite particle data as input to calculate the expected

average ionization profile across the precipitation region using the method described in Goldberg et al

[1984]. Comparison of the two results for a large number of cases should allow an estimation of the

fraction of the spacecraft relativistic flux which actually precipitates into the atmosphere.

Also during the first year (February 1999), we will begin acquiring VLF data at Fort Yukon and three

Fig. 5. Four Alaskan ground stations in the precipitation zone to be used for data acquisition. The phase and amplitude of the NPM, NLK, and NAA signals will be measured continuously at each ground station for the proposed program. Only the propagation paths from the NPM transmitter in Hawaii are shown. The use of spaced receiving stations allows high resolution determination of the electron density profile across the relativistic precipitation region.

additional ground stations on the west coast of Alaska. The three stations are being established as part

of a diagnostics system for the HAARP program under separate support. During most of the three year

period of our proposed program we will be able to use each station to monitor the phase and amplitude of

NPM, NLK, and NAA. A map with the location of the three stations plus Fort Yukon is shown in Figure

5, as are also the propagation paths to the stations from NPM. Measurement of the phase and amplitude

of NPM and NLK at the four stations will allow the separate determination of four parameter average

electron density profiles between L ~= 3.5 - 4 (Kipnuk - Mountain Village), L ~= 4 - 5 (Mountain Village

- Nome) and L ~ 5 - 7 (Nome - Fort Yukon). Thus the spatial resolution across the precipitation region

will be markedly improved by the four station network. Further increase in the cross L spatial resolution

may not be warranted since the SAMPEX data suggests that the precipitated flux may be describable by

as few as three parameters.

For example, Figure 6 shows theSAMPEX> 400 keVprecipitated flux over Alaska for four consecutive

days in 1992 as the flux was increasing. It is noteworthy that the maximum value of the flux for each day

is approximately the same, but occurs at a different L shell. The main change seen is that the zone of

precipitation enlarges each day. This set of curves could be reasonably characterized by three parameters,

such as the L shell of the maximum flux, the -3dB width of the distribution, and the -10dB width.

In addition to the four parameter electron density profiles across the Alaskan sector, we will also use

the NAA data to determine two-parameter average electron density profiles along the NAA path across

the North American relativistic electron precipitation zone. This will provide a measure of the overall

effect of relativistic electron precipitation on the North American continent.

The outcome of the first year’s effort will be a set of electron density profiles in the relativistic electron

precipitation region determined for a wide range of magnetic disturbance and a wide range of solar wind

conditions. These profiles will allow an accurate measure of the solar influence on the lower ionosphere

and middle atmosphere due to subauroral relativistic particle precipitation. In the second and third years

of our proposed program we will continue acquisition and analysis of VLF phase and amplitude data

from the four Alaskan stations shown in Figure 5 with the aim of building a deeper understanding of the

effects of relativistic electron precipitation on the lower ionosphere and middle atmospehre. Of particular

interest will be any changes in the characteristics of the precipitation and the ionization profiles that occur

as solar activity increases towards its maximum in the present solar cycle.

In addition we will continue comparison of the electron density profiles determined from the VLF

method with those predicted from satellite electron fluxes such as those of SAMPEX, UARS, and POLAR

in order to calibrate the spacecraft measurements for a wide range of solar conditions. The SAMPEX

data may include relativistic electrons which are locally trapped as well as those which are precipitated,

and thus we will calibrate the SAMPEX data with UARS and POLAR data whenever possible. Since

SAMPEX will be supported until its anticipated reentry in 2003, we expect that SAMPEX particle data

will be acquired until this time. Similarly POLAR particle data should be available through the upcoming

period of solar maximum. Since the spacecraft data generally passes into the public domain within a few

months of acquisition, we should have relevant spacecraft particle data available during the three year

period that our VLF observations will be made in Alaska.

1. Observations at Fort Yukon

During the period February - May, 1997, we operated a VLF receiver at Fort Yukon which measured

the amplitude and phase of the NLK, NAA, and NPM signals. These measurement were carried out as

part of the HAARP program in order the characterize the D-region over the HAARP facility during HF

transmission. In addition we operated the same receiver in a similar mode during September-November

1992 and March 1993 where we monitored both the NLK and NPM signals. Thus we already have in

hand ~ 6 months of data taken when SAMPEX and UARS were operational. The phase and amplitude of

the NLK, NAA, and NPM signals at Fort Yukon depends upon the electron density altitude profile along

the propagation path. During quiet times (such as day 179 in figure 2) when very little energetic electron

precipitation occurs, base line values of phase and amplitude for the three signals will be established

for the entire day. During active periods, when significant >400 KeV electron precipitation is occurring

along the propagation paths, the change of the signal amplitude and phase from their base line values can

be interpreted in terms of the magnitude of the electron precipitation flux and its spatial distribution, as

discussed below in section C. 2.

Figure 7b shows NLK signal amplitude data acquired in Fort Yukon in September-November 1992.

The quantity shown for each day is the NLK signal amplitude averaged over the three hour period 0600-

0900UT. A 27 day variation is apparent in the amplitude data, with deep amplitude minima (~ - 7dB)

occurring near days 280 and 307. Figure 7a. shows published [Baker et al, 1994] SAMPEX relativistic

electron precipitation data, as well as GOES 7 electron data. The SAMPEX data represents the daily

average flux. Two arrows in the upper panel indicate two intense relativistic electron precipitation

peaks that occurred in 1992. These precipitation peaks are coincident in time with the two amplitude

Fig. 6. SAMPEX > 400 keV precipitating relativistic fluxes on four consecutive days for orbits in the Alaskan sector. Each orbit exhibits approximately the same local time at L ~7. It can be seen that increases in the total precipitated flux occur predominantly as a result of the widening of the zone of precipitation and not as a result of an increase in the peak flux.

Fig. 7. Panel a) The upper plot shows the daily averages of the 2-6 MeV electron fluxes from the SAMPEX PET/ELO sensor for the L range 6.0 - 7.0. The lower plot shows the daily average flux of > 2MeV electrons measured at 6.6 > Earth Radii by sensors on the GOES - 7 spacecraft

Panel b) The amplitude of the NLK signal at Fort Yukon averaged over the three hour period 0600 - 0900 UT on each day for the period shown. A 27 day variation in this average amplitude is clearly apparent, and the amplitude minima coincides in time with the two electron flux maxima marked with arrows in the top plot of SAMPEX data.

minima evident in the Fort Yukon data. According to Figure 3, the VLF amplitude will have a minimum

whenever the relativistic electron precipitation has a maximum. Moreover, the predicted amplitude

decrease (~ - 7dB) agrees well with the observed amplitude decrease (~ - 8dB). Thus there is excellent

agreement between the two data sets in terms of event occurrence. This agreement represents a measure

of the precipitating component of the enhancements observed by SAMPEX, but a detailed quantitative

assessmentwould require the use ofSAMPEXflux data acquired along individual orbits near the NLK-FY

path, rather than the average daily flux values as shown in Figure 7.

2. Theoretical Modeling of the Effects on VLF Propagation of Ionization produced by Relativistic Electron Precipitation

Since the region in which subauroral relativistic electron precipitation takes place is known from

SAMPEX data, we face the relatively simple task of inverting the amplitude and phase measurements of

the VLF signals in order to determine the altitude profile of the ionization enhancements in the D-region

and middle atmosphere produced by relativistic electron precipitation.

An important component of this quantitative interpretation capability is a 3-D model of VLF propagation

and scattering in the earth-ionosphere waveguide in the presence of localized disturbances of

arbitrary size. This versatile model was developed in the context of a Stanford University PhD Thesis

[Poulsen, 1991] and the model and initial results have been described in several papers [Poulsen et al.,

1990; 1993a,b]. This 3-D model is an extension of a 2-D numerical propagation code (known as the

Long Wave Propagation Capability or LWPC) developed and tested by the U.S. Navy over a 20-year

period [e.g., Pappert and Ferguson, 1986]. The LWPC code is equivalent in complexity and versatility

to the well known Numerical Electromagnetic Code (NEC), which is commonly utilized in a variety of

practical applications [eg, Miller, 1980].

The new Stanford 3-D code is a fast, versatile code in which VLF signals propagating in the Earthionosphere

wave guide are represented as a sum of waveguide modes [Poulsen et al., 1993]. Variations

in ionospheric or ground conductivity along the great circle path between transmitter and receiver are

fully accounted for in the code. The waveguide mode properties are determined by solving Maxwell’s

equations in the vertical direction through the ionosphere, with the Earth’s magnetic field included.

Arbitrary ionospheric electron density and collision frequency profiles can be used at any point along

the propagation path. Thus, there is no difficulty in modeling electron density (or collision frequency)

perturbations with both horizontal and vertical structure.

The new VLF propagation code has already been successfully applied to a variety of geophysical

effects, including lightning-induced electron precipitation effects [Lev-tov et al., 1995], ionospheric

signatures of the auroral electrojet [Kikuchi and Evans, 1983; Cummer et. al., 1994, Fall AGU paper,

Best Student Paper Award; Cummer et al., 1996], high energy auroral particles effects [Cummer et al.,

1997, and ionospheric spots heated by VLF and HF transmitters [Rodriguez et al., 1994]. A detailed

inversion methodology has been developed as part of the Stanford University VLF D-region diagnostics

instrument recently developed for an Air Force program [Bell et al., 1995]. Most of these applications

involve relatively smaller VLF amplitude changes (0.1-2 dB), which are due to relatively small (<200 km

in size) ionospheric disturbances. As shown in Figure 3, the VLF signal changes caused by relativistic

electron precipitation effects are expected to be of order 10 dB, since the fluxes are high (thus leading

to substantial ionization changes) and since large portions of the propagation paths would be effected at

any given time. Detection and quantitative interpretation of such large signal changes is considerably

simpler than previous applications. Adaptation of the Stanford code to the three propagation paths shown

in Figure 1 can be readily accomplished through modest modifications.

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Outline

A. INTRODUCTION

B. SCIENTIFIC BACKGROUND AND QUESTIONS

Relativistic Electron Precipitation

Questions:

Approach:

Questions:

Approach:

C. THE RESEARCH

1. Observations at Fort Yukon

2. Theoretical Modeling of the Effects on VLF Propagation of Ionization produced by Relativistic Electron Precipitation

BIBLIOGRAPHY