Increased Precipitation without Increase in Potental

Published in Geophysical Research Letters, volume 24, pages 2933-2936, November 15, 1997.

LOCAL INCREASES IN AURORAL ELECTRON PRECIPITATION WHICH WERE NOT ACCOMPANIED BY A CORRESPONDING INCREASE IN THE ELECTRIC POTENTIAL OF THE AURORAL ELECTRON ACCELERATION REGION

W. Calvert
University of Massachusetts Lowell, Lowell, Massachusetts

D. A. Hardy
Phillips Laboratory, Hanscom AFB, Bedford, Massachusetts

Abstract. The Oedipus C rocket that was launched from Poker Flats, Alaska in November 1995 has detected significant local increases in the observed auroral electron precipitation flux during an auroral substorm which were not accompanied by a corresponding increase in the electric potential of the auroral electron acceleration region. The energy of the electrons which contributed to these increases in flux were also found to extend well below the electric potential energy of these electrons at the top of the acceleration region, thereby requiring a related loss in energy inside the acceleration region. This loss in energy must therefore be attributed to a wave instability which extracts energy from the electrons which are scattered into the loss cone inside the electron acceleration region to cause the discrete aurora during a substorm.

Introduction

Although Knight's theory predicts that the electric potential of the auroral electron acceleration region might determine the structure of the aurora by enlarging the loss cone for electron precipitation into the ionosphere, Lin and Hoffman [1982] have found that the observed electron precipitation during a substorm often tends to occur well inside the well-known inverted-V electron events which are found to accompany the aurora during substorm expansion. As pointed out by Calvert [1997a], this therefore contradicts the widespread assumption that latitudinally-localized electric fields in the high-altitude auroral zone cause the structure of the aurora during a substorm. It is therefore relevant to discuss the following observations of auroral electrons that were detected by the Oedipus-C rocket that was launched from Poker Flats, Alaska, at 06:38:17 UT on November 7, 1995.

Observations

As shown in Figure 1, Oedipus C reached an altitude of 824 km over the north coast of Alaska about 9 minutes after launch, at 06:46:54 UT. This rocket experiment consisted of two separate payloads that were aligned with the Earth's magnetic field and gradually separated in the direction of the magnetic field as a wire tether was deployed between its two sections [see James and Rumbolt, 1995; Eliuk et al., 1995]. Both of these payloads contained spherical electrostatic analyzers that were optimized for high spatial resolution in order to resolve the thin latitudinal structure of the discrete aurora which has been reported by Akasofu [1961], Maggs and Davis [1968], Kim and Volkman [1963], and Borovsky et al. [1991]. According to these observations, the latitudinal thickness of the discrete aurora during a substorm is found to vary from a few hundred meters to a few kilometers, whereas the average latitudinal thickness of the electric potential of the acceleration region, as measured by Newell et al. [1996], is found to be 28 to 35 km.

Figure 1. The trajectory of Oedipus C, showing the location of the local increases in electron precipitation that were detect ed by Oedipus C during the first half of the Oedipus-C flight.

This paper will report the observations of the electron analyzer which was directed looking upward in the aft payload of Oedipus C during the first half of the Oedipus C flight. Under the assumption that there is no other significant gain or loss in energy inside the acceleration region, the electric potential of the acceleration region, which according to Reiff et al. [1993] presumably occurs at an altitude of 2000 to 15,000 km above the auroral zone, can be measured from the energy of the electrons that are detected at low altitudes by Oedipus C.

This instrument detected incoming electrons in an eight-segment fan-shaped beam that was oriented to include the magnetic field and perpendicular directions. Each segment of this entrance beam was about 8 degrees wide by 17.5 degrees in pitch angle, and had an equivalent geometric factor of 8.5 x 10^-4 cm² ster. The electrons that were detected in these eight directions were sampled 10 times a second at 32 logarithmically-spaced energy steps from 10 eV to 20 keV. The energy resolution of these measurements was therefore about 30%, whereas the latitudinal resolution between adjacent energy sweeps, which is dictated by the rocket velocity in this direction, was about 100 m.

Figure 2 shows the aurora that was detected at a wavelength of 557.7 nm by the meridian scanning photometer at Poker Flats, Alaska, during the Oedipus-C flight. This figure shows the elevation angle of the aurora above the northern horizon, and the dotted line also shows the elevation angle of the footprint of the magnetic field line at Oedipus C at a height of 100 km. A discrete arc or group of discrete arcs along the trajectory of Oedipus C thus ought to show up as a lower edge in this figure, at the elevation angle of the active aurora at an altitude of approximately 100 km. Two such bands of aurora are found to occur in this figure, one beginning at about 06:25 UT which moved southward and faded out by the time Oedipus C was launched, and another beginning at approximately 06:32 UT which continued past the Oedipus C observations up to an elevation angle of about 17 degrees at 06:48 UT.

Figure 2. Aurora detected at Poker Flats, Alaska, where the dotted line also shows the elevation angle of the magnetic field line at Oedipus C at a height of 100 km.

These two bands of aurora, which showed the well-known equatorward motion of the discrete aurora during a substorm, are attributed to the occurrence of an auroral substorm during the Oedipus-C flight, and the aurora at lower elevation angles later on during this event are attributed to the subsequent poleward expansion of the aurora. It has also been found that Oedipus C was launched during a multiple-onset substorm that was detected by the magnetometer network in western Canada and Alaska, as well as by the onset of the auroral kilometric radiation that was detected by the Geotail satellite at a distance of 30 R_E in the post-midnight sector of the Earth's magnetosphere. As indicated in this figure, this behavior is consistent with the onset of an auroral substorm along an auroral L-shell which was approximately 300 km north of Poker Flats at 06:25 UT.

Figure 3.Electron energy measured by Oedipus C, showing a broad increase in the electric potential of the auroral electron acceleration region which can be attributed to the occurrence of an inverted-V electron event during these observations.

Figure 3 shows the electron energy that was detected by Oedipus C in the detector segment that was looking directly upward along the magnetic field. Since the angle of the loss cone at the altitude of Oedipus C during these measurements was approximately 56^o to 63^o, this figure shows the energy of electrons which precipitate into the ionosphere to cause the aurora. As shown in Figure 4, the only significant electron precipitation which was detected by Oedipus C during these measurements was found to occur between about 06:41:50 UT and 06:43:20 UT. Two unexpected local decreases in the elec tron energy were also found to occur in this figure, corresponding to local increases in the observed auroral electron precipitation flux at approximately 06:42:13 UT and just before 06:43 UT.

Figure 4. Electron energy spectrogram showing two local increases in the observed auroral electron precipitation flux at approximately 06:42:13 UT and just before 06:43 UT.

Figure 5 then shows the electron energy in the parallel and perpendicular directions for the first of these events, along with the electron precipitation flux that was calculated from the electron measurements in the top panel of this figure. This feature thus turned out to consist of three local increases in electron precipitation by a factor of three, six, and approximately nine, having a latitudinal width along the trajectory of Oedipus C which was approximately 300 m, 900 m, and 1.2 km.

Figure 5. Auroral electrons detected by Oedipus C, also showing in the middle panel no significant increase in the electric potential of the auroral electron acceleration region.

As shown in the middle panel of Figure 5, the electric potential of the acceleration region, which can be measured from the electron energy outside the loss cone during this event, was found to be approximately constant and equal to 3.3 ± 0.5 kV, whereas the energy of the electrons which contributed to these local increases in flux were found to vary from approximately 3.3 keV to significantly less than 3.3 keV, as shown by the spread in energy in the top panel of this figure.

The corresponding electron energy spectra for the third local increase in flux in Figure 5 are therefore shown in Figure 6 as a function of the time at 0.2-second intervals during this event. Since it is found to occur at the same energy as the electron energy outside the loss cone during these measurements, the peak energy in these spectra presumably corresponds to the electric potential of the acceleration region, thereby indicating a significant loss in energy inside the acceleration region.

Figure 6. Electron energy spectra measured by Oedipus C.

Discussion

The local increases in electron precipitation which are found to occur in Figure 5 presumably correspond to the discrete aurora during a substorm, since they are found to have the expected electron energy, precipitation flux, and latitudinal thick ness of the discrete aurora during substorm expansion. Similar local increases in the electron energy flux have also been reported by Evans [1974].

The broad peak in electron energy which is found to occur in Figure 3 is also attributed to a broad inverted-V electron event during these measurements, extending in latitude up to the apogee of Oedipus-C, as shown by the dotted line at 06:47 UT in Figure 1. Since the electron precipitation which is found to occur in Figure 4 turned out to be the only significant electron precipitation during this event, these measurements therefore also confirm the previous measurements of Lin and Hoffman [1982] which first suggested that the electric potential of the acceleration region does not match the structure of the aurora.

According to Knight [1973], in which it is assumed that the loss cone must be uniformly filled as a result of an isotropic Maxwell-Boltzmann electron velocity distribution at the top of the acceleration region, the resulting auroral electron precipitation flux turns out to be

where phi is the downward flux of electrons per unit area perpendicular to the magnetic field in the ionosphere, n and Wo are the density and average energy of the electrons at the top of the acceleration region, Bi is the magnetic field strength in the ionosphere, Btop is the magnetic field strength at the top of the acceleration region, e is the electron charge, m is the electron mass, and V is the total electric potential between the top and bottom of the acceleration region.

For Bi/Btop much greater than 3eV/2Wo, Equation (1) shows that the predicted electron precipitation flux increases ap proximately proportional to the electric potential of the acceleration region. This model for the aurora therefore requires an increase in the electric potential by a factor of three, six, and approximately nine in order to account for the observed in creases in flux which are found to occur in Figure 5, whereas the observed increase in the electric potential of the acceleration region, according to the electron energy in the middle panel of this figure, was found to be less than the 30% energy resolution of these measurements. These observations therefore confirm that the observed increases in electron precipitation flux cannot be attributed to a local increase in the electric potential of the electron acceleration region.

As pointed out by Calvert [1987, 1995, 1997b], the structure of the aurora during a substorm can be accounted for by local scattering into the loss cone by the cyclotron maser instability. Since this requires no local increase in the electric po tential of the acceleration region, this model for the aurora thus accounts for the observed lack of increase in the electric potential of the acceleration region in Figure 5. Moreover, since a wave instability converts part of the electron energy into an emitted wave, this model also accounts for the observed loss in energy which is found to occur in Figure 6.

This loss in electron energy also cannot be attributed to scattering into the loss cone above the acceleration region since the observed decrease in energy which is found to occur in Figure 6 exceeds the initial electron kinetic energy at the top of the electron acceleration region. These observations therefore also confirm that the predicted scattering into the loss cone must occur inside the auroral electron acceleration region. As pointed out by Borovsky [1993], it is also relevant to point out that no other theory of the aurora can account for the observed local increases in electron precipitation flux.

Conclusions

The Oedipus-C rocket has detected local increases in auroral electron precipitation which were not accompanied by a corresponding increase in the electric potential of the auroral electron acceleration region. These local increases in flux, which presumably correspond to the discrete aurora during substorm expansion, thereby contradict the traditional assump tion that localized electric fields cause the structure of the aurora during a substorm. The energy of the electrons which con tributed to these local increases in flux were also found to extend well below the electric potential energy of these electrons at the top of the acceleration region, thereby also requiring a related loss in energy inside the electron acceleration region which can be attributed to local scattering into the loss cone inside the electron acceleration region.

Acknowlegements. This work was supported in part by NASA contract NAS5-96020. The measurements in Figure 2 were provi ded courtesy of Dr. H. G. James, Communications Research Centre, Ottawa, Ontario, Canada, and the Geotail observations that were used to detect substorm onset were provided courtesy of Prof. H. Matusmoto, Radio Atmospheric Science Center, Kyoto University, Uji, Japan. Useful discussions with Dr. K. Hashimoto and Dr. Y. Omura of RASC are also gratefully acknowledged.

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(Received: April 8, 1997; revised August 27, 1997; accepted: September 10, 1997.)

W. Calvert, 219 Friendship Street, Iowa City, Iowa, 52245.

D. A. Hardy, Phillips Laboratory, 28 Randolph Road, Hanscom Air Force Base, Bedford, Massachusetts, 01731.