The GEOTAIL Plasma Wave Instrument (PWI) has provided high-frequency- and high-time-resolution details of the plasma wave morphology in and the instabilities generated near the boundaries in the magnetosphere and extending out to the deep geomagnetic tail. Strong Electron Plasma Oscillations (EPO) have been observed simultaneously with very low frequency (VLF) electromagnetic whistler-mode waves at the Plasma Sheet Boundary Layer - Lobe interfaces. Electromagnetic waves near the Upper Hybrid Resonance (UHR) frequency are occasionally observed near steep density gradients when intense EPO or UHR emissions are present. Isolated bursts of terrestrial myriametric (continuum) radiation in a limited frequency range typically from about 10 kHz to 50 kHz and lasting from tens of minutes to a few hours which we call "Continuum Storms" or "Enhanced Continuum" have been observed throughout the tail from as little as a few times per month to several times per day. Enhancements in the continuum radiation were usually closely associated with increased AKR activity indicative of substorms. Some continuum storms occur at constant delay times from the onset of AKR. Intense Electron Cyclotron Harmonic (ECH) emissions (also called "Totem Pole" emissions) have been observed coincident with the onset of moderately intense AKR and structured escaping continuum radiation. Often escaping continuum radiation appears to be generated at various boundaries in the tail. Type III solar radio burst triggering of AKR was occasionally evident. Broadband Electrostatic Noise (BEN) was strongly enhanced near boundary crossings. Frequently Extremely Low Frequency (ELF) electromagnetic waves are observed with enhanced BEN. Yet at other times, the ELF waves and BEN are strongly anti-correlated. On some days the Earth's bow shock was crossed several dozen times which produced many opportunities to study the plasma wave characteristics of both the upstream and downstream bow shock phenomena at varying penetration depths.
AKR measurements when GEOTAIL can view the nightside hemisphere provide excellent indications of substorm onsets. The ideal situation for substorm onset identification would include the simultaneous availability of multiple spacecraft observations of AKR, worldwide ground measurements, and POLAR imaging. Observations of continuum enhancements (which will be discussed later) also provide remote observations of substorms and the resulting plasma dynamics. The combination of the high time-resolution and remote sensing capabilities provided by the plasma wave measurements make them very important for studying the triggering, near triggering, and burstiness of substorms and related geomagnetic disturbances.
A 38-month data set of GEOTAIL plasma wave observations of AKR at 200 kHz and 500 kHz was used to examine the dependence of the angular distribution of AKR. Several important results confirmed and expanded on results from previous studies. One was the finding that the illumination region for the low frequency AKR range extends larger than that for the high frequency range. Such difference is basically explained by propagation. Another was finding that the equatorward extension of the illumination region for large Kp is caused by the equatorward shift of the auroral plasma cavity in the disturbed phase which should be related to the inward motion of the plasmapause just after the onset of substorms.
Several important new results were also found. One was the fact that the illumination region of AKR extends duskward as geomagnetic conditions become more disturbed especially for the low frequency range. This suggests the duskward extension of the AKR source. Lack of such feature at the high frequency range could be caused by insufficient density depression in the duskside auroral plasma cavity especially at lower altitudes. The primary generation mechanism presently considered for AKR, the electron cyclotron maser instability, requires a small plasma frequency to cyclotron frequency ratio. AKR is believed to be generated near the local electron cyclotron frequency such that high frequency AKR is generated at a lower altitude than the low frequency AKR. In the duskside plasmasphere, the electron density is enhanced so that the density in the auroral plasma cavity should be hard to decrease enough to satisfy the condition for the electron cyclotron maser instability. Therefore, generation of high-frequency AKR at lower altitudes is expected to be blocked on the duskside hemisphere.
A second new result was that the frequency of occurrence of AKR depends on observed time in UT which approximately corresponds to the longitude of the source especially for the high frequency range. The longitudinal dependence is evident in the same manner as shown in the optical auroral activity. This suggests that populations of energetic electrons are controlled by the altitude of the magnetic mirror point especially at lower altitudes. Another new result was that AKR is more active on the winter hemisphere especially for the high frequency range. Possible reasons include asymmetry of the population of precipitating electrons on the auroral field lines and insufficient density depression in the auroral plasma cavity on the summer hemisphere especially at lower altitudes which are most sensitive to ionospheric outflow. This study highlighted the fact that remote satellite observations can suggest the population of energetic electrons and the structure of the auroral plasma cavity on the auroral field lines which should be sensitive to the plasma density in the surrounding plasmasphere.
GEOTAIL PWI measurements along with those from the WIND WAVES experiment, the POLAR PWI, and GOES 8 along with CANOPUS and National Geophysical Data Center (NGDC) magnetograms have provided new information on terrestrial low frequency (LF) bursts. We have found that they are intimately related to auroral kilometric radiation (AKR) and are produced simultaneously with intense isolated substorms. The AKR is enhanced both in intensity and frequently in both the lower and upper frequency extent. This implies that the AKR generation region moves to both higher and lower altitudes during the isolated substorms. The absence of higher frequency AKR in some upstream observations of LF bursts can be attributed to propagation blockage by the earth and dense plasmasphere of the portion of the AKR generated at the lowest altitudes on the night side.
The GEOTAIL PWI has provided new details on Auroral Myriametric Radiation (AMR) which is a low-frequency radiation which correlates with simultaneously observed AKR emissions. Previous studies had reported on AMR in the 10 - 40 kHz frequency range. The GEOTAIL observations show that the frequencies of such correlated emissions become as low as 1 kHz and may not always be associated with AKR.
Broadband electrostatic noise (BEN) is a plasma wave phenomenon that dominates the spectrum throughout the tail especially in the presence of high speed flows and near boundaries of the different regions. GEOTAIL PWI studies were the first to show that most of the BEN wave forms observed by GEOTAIL in the Plasma Sheet Boundary Layer (PSBL) appear as a series of isolated spiky pulses which are termed "Electrostatic Solitary Waves (ESW)". This is crucial information for theorists trying to explain the generation of the emission. Comparison between the BEN observations and plasma measurements shows that the uppermost frequency of the ESW is closely related to the temperature of the flowing ions in the PSBL. We also find that the observed spike width of the ESW and the inter-pulse time-span change very rapidly on time scales ranging from a few milliseconds to a few hundreds of milliseconds, suggesting that the speed of the ESW potential changes very rapidly.
Narrowband electrostatic noise (NEN) has now been identified by the GEOTAIL PWI to consist of Quasi-monochromatic solitary waves.
GEOTAIL measurements in the Earth's bow shock have identified intense electromagnetic waves in which the magnetic components are at times comparable to the ambient magnetic field.
Verification of the Kennel and Petschek doppler shifted electron cyclotron resonance whistler-mode wave-particle interaction was obtained for chorus emissions detected by the GEOTAIL PWI and correlated with growth rates calculated from electron distribution functions measured by the GEOTAIL Comprehensive Plasma Instrument (CPI). Three-dimensional electron velocity distribution functions and plasma wave data acquired from GEOTAIL in the Earth's outer magnetosphere were used to correlate linear cyclotron growth rates and the activity of chorus to show that the nonlinear chorus emissions are generated initially by a linear cyclotron interaction. The GEOTAIL Plasma Wave Instrument has also observed chorus triggered by ULF emissions near the dayside magnetopause for a period of eight hours. The WFC receiver data were used to determine the characteristics of the chorus emission, such as the wave normal direction, polarization, refractive index and Poynting flux. The wave normal directions of the chorus were not always parallel to the earth's magnetic field.
Electron plasma oscillations (Langmuir waves) and their second harmonic (2Fp emissions) occur in the electron foreshock region upstream of the Earth's bow shock. GEOTAIL has provided both remote sensing and in-situ observations of the terrestrial foreshock to study 2Fp emissions. The geometry of the 2Fp source region was determined by three types of statistical remote sensing analysis: mapping of the 2Fp flux, timing analysis of bifurcation phenomena associated with density discontinuities in the solar wind, and propagation direction determined by spin modulation. Three major points were found. The first was that the 2Fp source region is on the tangential field line to the bow shock where strong Langmuir waves occur. This provides direct evidence that the 2fp emission is generated from intense Langmuir waves. The second was that both Langmuir waves and 2Fp emissions are not strong around the contact point of the tangential field line to the bow shock where acceleration of electrons is expected. This suggests that sharp electron beams are not formed well enough to generate intense Langmuir and 2Fp waves because flight time in the region close to the contact point is too short to develop electron beams through the velocity filtering. The third was that the distance of the central position of the source region from the Earth is generally up to 40 Re. This suggests typical flight length of free energy consumption in the electron beam is sufficient to excite 2Fp emissions. These results are consistent with triangular analysis of simultaneous GEOTAIL/PWI and WIND/WAVES observations of 2Fp emissions.
The detection of intense low frequency electromagnetic waves is usually indicative of the presence of strong currents. Observations of the low frequency MCA magnetic field data have been used in the regime identification study reported in Eastman et al., [1997]*.
*EASTMAN, T. E., S. P. CHRISTON, G. GLOECKLER, D. J. WILLIAMS, A. T. Y. LUI, R. W. McENTIRE, E. C. ROELOF, T. DOKE, L. A. FRANK, W. R. PATERSON, S. KOKUBUN, H. MATSUMOTO, H. KOJIMA, T. MUKAI, Y. SAITO, T. YAMAMOTO, and T. K. TSURADA, Identification of Magnetospheric Plasma Regimes from the Geotail Spacecraft, to be submitted to J. Geophys. Res., 1997.
Plasma wave measurements help identify and study features associated with plasmoids, flux ropes, and other high speed flow events in the Earth's deep geomagnetic tail. The most common are broadband electrostatic noise (BEN) and very low frequency electromagnetic emissions. The frequency of occurrence and intensity of both of these emissions are strongly dependent on the plasma flow speeds and the plasma temperatures. The BEN can be quite variable throughout an event. Sometimes the BEN is very intense at very low frequencies but only extends up to a few hundred Hz. At other times the intensities at the lower frequencies are reduced but the BEN extends up in frequency to many kHz (typically near the plasma frequency). Occasionally the electrostatic bursts occur as intense discrete rising tones or falling tones. The very low frequency electromagnetic emissions commonly observed are strongest at the lowest detectable frequency of 5 Hz and usually extend up to only a few tens of Hz. Isolated narrow-band electromagnetic emissions in the frequency range from 10's of Hz to around a hundred Hz are sometimes detected. Enhanced Langmuir waves (electron plasma oscillations) similar to those detected near the earth's bow shock are observed before and after (and occasionally during) the passage of plasmoids in the tail. At the bow shock they are attributed to reflected and accelerated electron beams. This would suggest that the plasmoids have their own boundary and do not remain connected to the earth as they move down the tail. Electrostatic emissions near the electron cylotron frequency are also occasionally observed. The mixture of broadband and narrowband electrostatic emissions and extremely low frequency electromagnetic waves observed within plasmoids indicates the presence of numerous and varying free energy sources.
By comparing the timing of AKR onsets and plasmoid entries observed by the GEOTAIL spacecraft we were able to estimate the location of the reconnection line and plasmoid size in the geomagnetic tail. We first compared AKR onset events with high energy particle observations at geosynchronous orbit. We determined the plasmoid ejection (reconnection) time by the AKR enhancement only when it corresponded to energetic particle enhancement within five minutes. The traveling time of the plasmoid from the X-line to the spacecraft was calculated by the difference in time of the AKR onset and that of the plasmoid encounter with GEOTAIL. Assuming the plasmoid propagates with the Alfve'n velocity in the tail lobe as MHD simulations predict, we estimated the location of the reconnection line in 11 events. The results showed that the most probable location of the plasmoid edge is distributed around x = -60 Re in GSE coordinates. The estimated size of the plasmoids ranges from 10 to 50 Re in the x direction. If we apply this result to the alternative plasmoid model in which the evolution of the tearing instability causes the generation of plasmoids, the X-line should be approximately at x = -35 Re.
Continuum Storms (also referred to as enhanced continuum) observed by the GEOTAIL Plasma Wave Instrument in the Earth's geomagnetic tail are associated with moderate intensity geomagnetic disturbances. The negative bays associated with them are typically between -200 nT and -300 nT. The observations of structured and multiple continuum enhancements by GEOTAIL during substorms offer remote tracking of injected plasmas. The continuum is produced at high density gradients found at the plasmapause and magnetopause as the injected plasmas impinge on them. We have interpreted the observation of enhanced continuum with more than one rising feature with different slopes as being due to multiple paths for the injected electrons.
GEOTAIL has confirmed earlier observations that low frequency electrostatic waves are enhanced near the neutral sheet but exactly at the neutral sheet crosssing almost all wave activity ceases. The enhanced waves near the neutral sheet could provide some or all the anomalous resistivity expected there.
Plasma wave measurements of upper hybrid resonance frequency emissions in the magnetosphere, plasma frequency emissions in the solar wind and magnetosheath, and plasma frequency cutoffs in the tail lobe, plasma sheet, and boundary layer regions provide accurate measurements of plasma density independent of spacecraft charging. In fact, particle experiments use the PWI derived density to calibrate their instruments.
Recent wave and particle measurements made by instruments onboard the GEOTAIL and POLAR spacecraft have been compared for times when both spacecraft were crossing the same magnetic field lines. GEOTAIL was skimming the magnetopause, and POLAR was at or near one of the northern or southern polar cap boundary layers. The data taken during these times suggest that POLAR instrumentation is observing the effects of high altitude heating of upflowing ionospheric ions, through wave-particle interactions. GEOTAIL observes these same ions after they have propagated into the magnetopause boundary layer through heating and acceleration. These observations, together with solar wind plasma and interplanetary magnetic field measurements from WIND, images of the footprints of the magnetic field from POLAR, and ground-based, remote sensing measurements, are being studied in order to gain a further understanding of the magnetopause boundary layer and the transport and acceleration processes that take place in and near the magnetopause region.
Interesting correlations have been found between the observation of continuum storms throughout the Earth's geomagnetic tail by the GEOTAIL PWI and the CANOPUS ground magnetometer data. During the nearly five years that GEOTAIL has been in operation, PWI has detected numerous continuum storms in the geomagnetic tail at distances ranging from 10 Re to 210 Re. Data from the CANOPUS ground magnetometer network has frequently shown good correlation of the onset of the continuum storms with either an increasing CU, decreasing CL, or increasing difference (CU-CL) in the CANOPUS Key Parameter data. The best correlations do occur when the CANOPUS chain is near midnight local time. Studying the time differences in the onsets allows us to determine that in some instances the continuum source is near the earth while in other instances the sources may well be near the magnetopause boundary far down the tail. Some of the continuum storms occur near in time to the passage of a plasmoid or flux rope over the spacecraft. Some of the continuum storms exhibit discrete structures indicative of non-local plasma frequency harmonics being their source. We are carefully studying individual station magnetograms in order to understand the relationships of the near earth plasma and its dynamics to the characteristics of the various continuum storms. We have found the somewhat unexpected result that the continuum storms were typically associated with modest negative bays of the order of only about -200 to -300 nT. R. R. Anderson is leading this study of with valuable assistance from his GEOTAIL PWI colleagues in Japan and Gordon Rostoker in Canada.
One subject of great interest to scientists trying to understand substorm dynamics is the plasmoid or flux rope. The PWI and MCA observations of plasma waves related to plasmoids and flux ropes are adding much to our understanding of these phenomena.
Many studies related to BEN and NEN are in progress. A paper examining the relation between electrostatic solitary waves and hot plasma flow in the plasma sheet boundary layer has already been published [Kojima et al., 1994]*. The possible association of the low frequency electromagnetic waves and the various electrostatic waves are also being studied. The MCA measurements are especially important for these different studies because they extend to a lower frequency than the other PWI receivers and because they provide much better time resolution than the SFA and much more complete temporal coverage than the WFC.
*Kojima, H., H. Matsumoto, T. Miyatake, I. Nagano, A. Fujita, L. A. Frank, T. Mukai, W. R. Paterson, Y. Saito, S. Machida, and R. R. Anderson, Relation between electrostatic solitary waves and hot plasma flow in the plasma sheet boundary layer:GEOTAIL observations, Geophys. Res. Lett., 21, 2919-2922, 1994.
W. W. L Taylor is leading a morphological study with the goal of imaging the magnetosphere and its tail using plasma waves observed with the GEOTAIL PWI MCA experiment. A statistical study of the multichannel analyzer electric and magnetic field data during the first three years (September 1, 1992 to August 31, 1995) continues. The University of Iowa has provided the programming support for the statistical survey. The results will help the viewer visualize the intensities of the plasma waves throughout the magnetosphere. The primary goal of this study will be to reveal visually the generation and propagation regions of plasma wave emissions throughout the parts of the magnetosphere that GEOTAIL traverses. In addition, this study and similar studies of plasma wave data from spacecraft in complementary orbits will allow techniques to be developed to map plasma densities throughout the magnetosphere using radio sounding. For this study, the magnetosphere was divided into cubical regions varying in size from 2 Re on a side to 10 Re on a side, depending on its location in the magnetosphere. GEOTAIL passed through over 2000 of these regions during the three years studied. More than 1.9 E 8 measurements in each of the 34 analyzer channels were made. The number of times each of the 256 possible spectral densities was measured in each of these regions in GSM coordinates was determined for each frequency (electric field - 5.62 Hz to 311 kHz, magnetic field - 5.62 Hz to 10 kHz). This massive statistical data set holds a treasure of information about not only plasma waves, but also about magnetospheric regions and boundaries and the wave-particle interactions that control much of the magnetosphere. The figures plotted from this study help identify the spatial distributions of the plasma wave emissions throughout most of the magnetosphere.
The results of this study will be used to develop techniques to map plasma densities throughout the magnetosphere using radio sounding. In addition, the distribution of the electric and magnetic field spectral densities will be very useful in determining the operational scenarios for radio plasma imaging for the IMAGE space physics mission proposed to NASA as a MIDEX project. As an example, continuum radiation may limit, but will certainly not eliminate, the coverage of magnetopause sounding for IMAGE. To determine the extent of this limitation, a joint study of GEOTAIL, POLAR, WIND and CRRES PWI data has just begun. The first periods of interest have been identified and the appropriate investigators have agreed to participate in the study.
At a joint GEOTAIL/WIND workshop held in Honolulu, Hawaii in May, 1995, several new studies were initiated between the experimenters on the GEOTAIL and WIND and IMP-J spacecraft, ground observatories, and theorists, simulators, and modelers. The Low Latitude Boundary Layer group chaired by Terry Onsager and Masato Nakamura solicited data for several events which would be made available to all participants on the World Wide Web. We provided MCA color spectrograms. The MCA data in several instances were able to identify narrow regions of plasma adjacent to the boundary.
The GEOTAIL PWI experiment along with the POLAR PWI experiment and the WIND WAVES experiment can provide much information on the dynamics of substorms from their remote observations of AKR, Low Frequency (LF) bursts, and continuum radiation emissions received at the three spacecraft in various locations in the magnetosphere. A strong correlation between AKR and LF bursts and geomagnetic activity has already been established. Multiple spacecraft observations can now be used to extract inferred plasma dynamics. The frequency limits on the observed AKR and LF bursts are indicative of the altitude of the generation regions. However, these limits could also be affected by high density plasmas between the source and the observing sites. Careful analysis of observations from multiple spacecraft allows us to identify the density and the motion of the intervening plasmas. Only now do we have sufficient spacecraft to begin separating out the generation and propagation affects. Continuum enhancements related to substorms are believed to be due to the dynamics of the injected particles. Simultaneous observations of continuum enhancements with different time dispersion profiles indicate that the injected particles follow diverse paths. These observations need to be compared with data obtained from the POLAR imagers, the magnetometer and particle data from the geosynchronous satellites, the CANOPUS instruments, and other ground magnetometer data. In some cases the in situ measurements from the magnetometer and particle experiments on GEOTAIL, POLAR, and WIND will also provide useful data regarding the substorm dynamics. Imaging of the Energetic Neutral Atoms (ENA) being carried out using the POLAR energetic particle detectors could also substantiate the inferences made from the remote plasma wave observations
A very important reason to extend GEOTAIL and ISTP into the solar maximum period is to identify and compare the significance of various substorm triggering processes. Changes in the IMF and solar wind density and velocity have long been the primary considerations. However, many substorms occur with no obvious change in any of the solar wind parameters. Low frequency (LF) bursts observed by the various ISTP/GGS plasma wave and radio receivers occur simultaneously with intense isolated substorms. Desch and his colleagues on WIND found that the LF bursts were well correlated with high solar wind speeds. However, many of the LF bursts we have observed on GEOTAIL occur during moderate solar wind speeds and periods having no obvious change in any of the solar wind parameters. Although a number of scientists remain skeptical, some triggering of AKR by solar type III radio bursts has been substantiated. Calvert has even suggested that waves (AKR) scattering electrons into the loss cone could be the initiator for some substorms. A more complete understanding of the triggering of substorms should definitely contribute to the space weather initiative.
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