Auroral Density Cavities

L. Eliasson and R. Lundin

Swedish Institute of Space Physics

Box 812

SE-981 28 Kiruna, Sweden


Macro- and meso-scale plasma density cavities are associated with transverse heating /energization of ionospheric ions in the topside ionosphere. Inside these depletion regions the density may decrease by more than two orders of magnitude. The plasma density inside the cavities may reach values below 10 cm-3 at an altitude of about 1700 km. This suggests that transverse ion heating is a very strong mechanism for plasma density depletion. The transverse ion energization is associated, but not always collocated, with large-scale inverted V electron acceleration regions. In fact, most of the transverse energization is not collocated with inverted V electron precipitation, but rather with regions characterized by wide-energy field-aligned electron beams. The orbit of the Freja satellite, with an inclination that allows it to traverse the auroral oval tangentially and stay for minutes on field lines connected to the auroral energization region at about 1700 km altitude, was ideal to explore the phenomenological connection between ion heating/outflow and auroral ionospheric plasma density cavities. We will also discuss the Freja wave measurements and make a comparison with observations from other satellites and ground based instrumentation.


Regions with decreased ionospheric density were observed already in the sixties in, e.g., Alouette topside sounder data (Muldrew 1965). The notion of an auroral plasma cavity was introduced by Calvert (1981) on the basis of Hawkeye data. The plasma depletion was found to extend downward to 2000 km altitude. Figure 1 shows the auroral plasma cavity as a function of geomagnetic latitude and radial distance, superimposed on dipole field lines (from Calvert, 1981).

Observations within auroral plasma cavities have been made with several other spacecraft, e.g., ISIS 1 (Benson and Calvert, 1979), S3-3 (Temerin et al., 1981), DE-1 (Persoon et al., 1988), Viking (Lundin and Hultqvist, 1989) and Freja (Lundin et al., 1994). The auroral density cavity is related to high wave activity, non-linear plasma instabilities, and AKR (Benson, 1985; Pottelette et al., 1988; Bahnsen et al., 1989; Hilgers, 1992; Pedersen et al., 1992; Roux et al., 1993; Benson, 1995). At mid-altitudes the cavities are characterized by very low plasma density. The minimum density frequently reaches values below 0.3 cm -3 in the altitude range 2 - 4.6 Re. It was also found that density depletion regions are associated with upgoing ion beams. Haerendel (1989) and others discussed the existence of large holes in the topside ionosphere in terms of deeply protruding cavitation processes in the lower edge of the auroral energization region.

The processes by which ionospheric plasma gets heated and expelled out into the magnetosphere are essentially nonthermal. The outflow of ionospheric plasma is quite substantial. Chappell et al. (1987) estimated it to be 2-4 kg/s. Indeed, already Block and Fälthammar (1968) proposed that the effect of field aligned currents would lead to a substantial erosion of the topside ionosphere.

We present examples of how the transverse energization process creates large depletion holes in the ionosphere. We show that large-scale density depletion does not necessarily follow the general "inverted V" related field-aligned acceleration pattern, but may extend over broader regions with transverse ion energization. The small-scale size, 10-50 m lower hybrid cavities, will not be covered in this report.


The Freja satellite, with an apogee of 1750 km, rarely encounters the mid-altitude parallel auroral energization region. It is, however, instead uniquely suited to study the impact of the auroral energization process on the structure of the topside ionosphere. More informa tion on the Freja mission can be found in, e.g., a special issue of Space Science Reviews, Vol 70, Nos. 3-4. 1994.

The Viking piggyback launch with the Spot spacecraft put some restrictions on the initial para meters of the orbit but made Viking extremely good for studies of dayside phenomena. More information on the Swedish Viking project can be found in, e.g., Hultqvist, 1990.

Some orbit and instrument information for the Freja and Viking projects are given below.

Orbit:		Inclination         63.0°
		Apogee altitude     1756 km
		Perigee altitude     601 km
		Orbital period       109 min
Launch:		LM-2C rocket from JSC, China on 6 October 1992
Attitude:	Spin stabilized 10 rpm
		Sun oriented (± 30°)
		Magnetic control
Size:		2.2  x  0.5 m
Mass:		255.9 kg  at launch
		214.0 kg  in orbit
Payload weight:	73.1 kg (including 21.6 kg stiff booms and wire booms)
End of operation:	October 1996

F1  Electric Fields, 3 pairs of wire booms, 20 m tip-to-tip
		     2 comp. of E  up to 6000 samples/s
Göran Marklund, Alfvén Lab., KTH, Stockholm, Sweden

F2  Magnetic Fields, Triaxial flux-gate on 2 m boom
		     3 comp. of B, 128 samples/s
Lawrence Zanetti, Johns Hopkins Univ., APL, Laurel, MD, USA

F3H  Particles, 2D magnetic electron spectrometer
		2D distr. 0.1-115 keV, 100 samples/s 
		2D Ion composition spectrometer
		3D distr. 0.001 - 5 keV in 3 s
Lars Eliasson Swedish Institute of Space Physics,IRF-K, Kiruna, Sweden

F3C  Particles, 2D ion/electrons on 2 m boom
		3D distr. of cold plasma (<300 eV) >100 samples/s
(Brian Whalen), Dave Knudsen, Univ of Calgary, Canada

F4  Waves, wire booms + 3 axis search coil
		 E, B, n Waves, 1 Hz - 4 MHz
Bengt Holback Swedish Institute of Space  Physics,IRF-U, Uppsala, Sweden

F5  Auroral Imager, 2 UV  CCD cameras
		    Auroral images every 6 s    
John S Murphree, University of Calgary, Canada

F6  Electron Beam, Three electron guns
		   3 components of E, 100 samples/s
Götz Paschmann, Max Planck Institut für  extraterr. Physik, Garching, Germany

F7  Correlator, 2D electron spectrometer
		0.01 - 20 keV, correlation F4
Manfred Boehm, MPE Garching (now at Lockheed Martin Palo Alto Res. Lab., USA)

Project Scientists          
Rickard Lundin, Swedish Institute of Space Physics, Kiruna, Sweden
Gerhard Haerendel, Max Planck Institut für  extraterr. Physik, Garching, Germany   

Orbit:		Inclination		98.8°
		Apogee altitude		13530 km
		Perigee altitude	817 km
		Orbital period		262 min
Launch:		Ariane 1 rocket from Kourou on 22 February 1986
Attitude:	Spin stabilized 3 rpm
		Cartwheel mode
Size:		1.9 x 0.5 m (motor extended further)
Mass:		538 kg at launch
		286 kg in orbit
Payload weight:	44.7 kg (68 kg including booms and antennas)
End of operation: May 1987

V1     Electric field experiment
       Lars Block, Alfvén Lab., KTH, Stockholm, Sweden

V2     Magnetic field experiment
       Tom Potemra, Johns Hopkins Univ., APL, Laurel, USA

V3     Particle experiment
       Rickard Lundin, Swedish Institute of Space Physics, Kiruna, Sweden

V4L     Low frequency wave experiment
       Georg Gustafsson, Swedish Institute of Space Physics, Uppsala Division, Sweden

V4H     High-frequency wave experiment
       Axel Bahnsen, Danish Space Research Institute, Denmark

V5     Auroral UV-imaging experiment
       Cliff Anger / Sandy Murphree, Univ of Calgary, Canada

Observations in low plasma density regions with the Freja satellite

Freja ion (O+) and electron energy-time spectrogram plots (top and bottom panel), total electron flux/counts (MATE, 0.2 - 100 keV), and Langmuir probe data of the ambient plasma density for orbit 1954 are shown in Figure 2. Ions are accumulated for pitch angles 45°-135° and electrons for pitch angles 0°-20°. The figure illustrates a tangential traversal of a large-scale ionospheric plasma depletion region with transverse accelerated ions (TAI). The transverse ion energization region (04.19 - 04.27 UT) is associated, but not exactly collocated, with a large-scale "inverted V" electron energization region (04.16 - 04.20 UT). The electron acceleration is in this case fairly irregular with many small scale "inverted V" structures embedded in the large-scale structure. At the poleward edge of this slanted traversal of the oval (04.22 UT), there is a marked change in pattern with more structured low-energy electron fluxes associated with enhanced energies of the energized ions. This is, in fact, associated with the lowest ambient plasma density of the region traversed by the spacecraft. A striking feature of this plot is that the overall peak energy of energized ions correlates well with the decreased density measured by the Langmuir probe. The spacecraft potential remained almost constant within the energization region, and only a modest increase by a few volts of the satellite potential occurred near 04.22 UT. Thus, the Langmuir probe should provide accurate plasma density measurements for this pass.

The Freja ion (H+ and O+) and electron energy-time spectrogram plots and Langmuir probe data of the ambient plasma density for orbit 1798 in Figure 3 demonstrates the close correlation, sometimes even in the fine structure, between the plasma density measured by the Langmuir probe and the maximum energy of heated ions. This pass shows the traversal of a meso-scale plasma depletion cavity with densities down to 10 cm -3 and with localized transverse ion energization. However, in this pass the density depletion between 09.07 - 09.10 UT is not connected with any significant ion acceleration at the equatorward side (09.07 - 09.08 UT). There are several possible reasons for this discrepancy. One is a poleward motion of the plasma energization region, leaving a dens ity depletion region behind. Another is related to temporal variations in the plasma energization region, i.e. the slope of the density is the remaining "imprint" of a preexisting heating process. Yet a third could be that transport processes bring plasma to the poleward side, thus depleting plasma also from the edges. Whatever the reason, the example demonstrates that large scale plasma density cavities in the topside ionosphere may extend outside the region where plasma energization is temporarily observed.

The apparent bulk ion outflow of moderately heated ionospheric plasma is associated with strong ionospheric plasma density dropouts inside the "inverted V" electron accele ration region (09.04 - 09.08 UT), with electrons accelerated up to 1-2 keV. In this case the transverse energization is small, of the order 10 eV or less. The parallel component is about the same, 10 eV. See Lundin et al., 1994 for more details on large-scale plasma density depletions observed by the Freja spacecraft.

Observations in low plasma density regions with the Viking satellite

Conclusive evidence for the mid-altitude energization of ions and electrons came from the S3-3 satellite, traversing for the first time with appropriate instrumentation the core of the acceleration region. The discovery of perpendicular electric "shocks" and weak double layers introduced further complexity to the perhaps too primitive conjecture of electrostatic field-aligned potentials along magnetic field lines. Instead of constituting a smooth poten tial well structure the electric field measurements implied considerably more temporal /spatial variations acting within the acceleration region.

The launch of DE-1 in 1981 into a somewhat higher orbit (apogee 23 000 km) than S3 -3 (apogee 8 000 km) added further discoveries from the auroral energization region. For instance, so called "electron conic" signatures (Menietti and Burch, 1985) are features of the electron distribution function that cannot be attributed to a simple electrostatic accele ration.

Similarly, the launch of Viking in 1986 and the very detailed measurements of particles and fields carried out during the about 1 year lifetime of the spacecraft marks another landmark for the understanding of the mid-altitude auroral energization region (Lundin and Eliasson, 1991). With Viking it has been possible to study the properties of the particle energization in more depth.

There are, e.g., several examples in the Viking data of elevated ion conics reaching an acceleration up to 40 keV, the upper energy limit of the instrument. One of these events can be seen at about 20:13 UT in Figure 4. The electrons are characterized by bi-directional field-aligned distributions with energies up to several keV.

An example of a strong correlation between ion beam and electric field fluctuations is shown in Figure 5. The roughly perpendicular electric field component is shown in the middle frame. Note the fluctuations during the periods when ion beams (062815-062915 UT) and ion conics (from 063025) are observed. The electric field fluctuations are observed in regions with depleted plasma density as indicated by the decreased value of the floating ground potential and the estimated electron number density (Lundin et al., 1990). The electron data (uppermost panel) shows an "inverted V" type of electron distribution between 06.27.30 and 06.30.30 UT. The electrons are accelerated above Viking to about 8 keV except in the center of the region where about 4 keV is gained above the satellite and 4 keV below (estimated from the ion data). The total potential drop is thus about the same during the main part of the arc crossing but the altitude of the acceleration is changing. Notice also that there are almost no electric field fluctuations when the acceleration region is above Viking.

Figure 6 shows an example of Viking particle data (top two panels) and electric field data (E2bsp is the electric field perpendicular to B in the spin plane and V fg [ proportional to log(neTe1/2)] is the floating ground potential) from a northern eveningside oval pass (Lun din et al., 1990).

It illustrates the good correlation between low-frequency electric field fluctuations (LEFs) and the occurrence of ion beams and conics. Notice for instance that the largest fluctuations are associated with a time period when ion conics were present. The correlation study (Lundin et al., 1990) demonstrated that the ion conic temperature showed the best correlation with LEFs. The higher the power of the LEFs the higher was the temperature of the conics. The fact that the best correlation between LEFs and ion temperature is for conics implies a local process. Furthermore there is a lack of LEFs below the acceleration region. Thus, LEFs must be produced at altitudes above or within the acceleration region. It is quite obvious that the energy must come from higher altitudes. Figure 7 shows scatterplots of the correlation between the ion conic temperature and the electric field power spectral density at three frequency intervals.

Electron angular distributions peaked at oblique angles to the magnetic field, electron conics, are frequently found in the Viking data at all magnetic local times (Eliasson et al., 1996). The ion and electron angular distributions observed by Viking indicate that a parallel electric field is present below Viking during electron conic events. Electron conics were detected in the Viking data in about every third orbit that was studied. Their frequency of occurrence in altitude maximizes in the upper part of the acceleration region, between 10.000 and 11.000 km. Wave observations show the presence of both low frequency waves and waves close to the electron gyro frequency. Acceleration in a fluctuating (1 Hz) parallel electric field has been suggested (Eliasson et al., 1996) as a likely mechanism to create the electron conics observed by Viking. The period of the fluctuations is comparable to the travel time, below the acceleration region, of the electrons forming the conic distri bution.

The wave experiments on board Viking observed low frequency electric field fluctuations and upper hybrid waves for all events that we have studied in more detail. Figure 8 shows an afternoon crossing of the poleward part of the auroral oval at about 10.000 km altitude. The uppermost panel shows electric field data from the V4H high frequency wave experi ment in the frequency range 10 to 128 kHz. Estimates of the electron gyrofrequency, the electron plasma frequency, and the lower hybrid frequency are indicated in the three upper panels. The second panel from the top also show wave emissions in the frequency range 1 to 500 kHz. The third panel shows emissions in the 0 to 400 Hz range. The line at about 50 Hz indicates the proton gyro frequency. Electron count rates between 10 eV and 40 keV, ion count rates in the energy interval 40 eV to 40 keV, and the pitch angle of the particle observations are shown in the bottom three panels. The electron conics are seen between 0.45.00 and 0.46.30 UT. The horizontal lines in the uppermost panel are caused by interference. The vertical lines appearing every minute are caused by an active sounder experiment. It can be seen that the electron conics are correlated with impulsive broad band noise both in the 10 kHz range (most likely hiss emissions) and in the 0 to several 100 Hz frequency range as well as with waves close to the electron cyclotron frequency (EC), most likely upper hybrid (UH) waves. The EC/UH waves are, however, more localized than the hiss emissions and seem to be better correlated with the electron conics. The general wave intensity increases in the region of electron conics as well as in a later region were the particle instrument observed some signatures of ion conics. The region with obvious electron conics is associated with the lowest electron densities 1 - 3 cm -3. This fact can be important for the growth of waves close to the electron gyro frequency.

Transverse ion heating observed with the Freja spacecraft

The Freja orbit is very well suited for studies of the interaction between the cold ionospheric and the hot magnetospheric plasma. The 63° inclination frequently gives tangential travers als of the auroral oval, covering an extended longitude range. Large-scale "inverted-V" structures are covered in longitude by up to 5 hours in magnetic local time. Such traversals display at times a remarkable stability of the auroral acceleration process, but at other times also highly dynamical features. Ions may become accelerated transverse as well as parallel to the geomagnetic field. Ions heated perpendicular to the magnetic field will appear as ion conics at higher altitudes due to the geomagnetic mirror force. Such accele ration has been detected at virtually all altitudes above 400 km on auroral field lines. No consensus has been developed as to the fundamental mechanism for their acceleration (e.g., Eliasson et al., 1994). The result of ion heating transverse to the geomagnetic field is frequently seen in the Freja data. Heavy and light ions are heated to characteristic energies from a few eV up to about 50-100 eV. However, transversely heated ions are seen up to the high energy limit of the hot plasma instrument, 4.5 keV.

The ion heating event in Figure 9 is observed in a region with intense electron precipitation at an altitude of about 1700 km in the eveningside auroral oval. The MATE 3-100 keV electron data (bottom panel) show anisotropic and variable electron distributions indicating electron energization to more than 10 keV. The TICS mass spectrometer observations of oxygen and hydrogen ions (0.5 eV to 4.5 keV) are displayed in panel three and four. The vertical stripes after about 2.34 UT in the ion data occurring twice per satellite spin indicate perpendicular ion heating. The two top panels show the B-field and E-field wave data in the frequency range 0-3 kHz. There is an intensification of the wave activity between 02.34.00 and 02.37.30 UT, i.e. simultaneously with the electron precipitation and ion heating. The most intense waves are found above 600 Hz, which is interpreted as the lower hybrid frequency. The wave amplitude integrated over one kHz above the lower hybrid frequency is up to and above 100 mV/m. There are also broadband emissions with lower amplitudes up to and around the proton gyro frequency associated with the ion heating. The data gap seen at 2.35 UT is due to a change of antennas. Due to the Freja orbit, observations are made below or in the lower part of the acceleration region. This explains the apparent absence of upgoing ion beams with energies above tens of eV in the Freja data.

An example of ion heating in the early morningside auroral oval associated with precipitating ions can be found in Figure 10. The precipitation is strongly dominated by protons (4th panel). The ion heating occurs in a region were the electrons show central plasma sheet characteristics with no discrete features, at least not in the energy range covered by the MATE (100 keV) and TESP (10 eV and 25 keV) detectors (bottom panels). There is, however, an auroral structure with electrons accelerated to about 1 keV adjacent to the proton precipitation region. The B- and E-field emissions up to 10 kHz are shown in the top two panels. There are emissions around one kHz, which probably is the lower hybrid frequency. There is a very good correlation between the appearance of waves in the lower hybrid range of frequencies and precipitating protons. Note also that when the electron intensity increases the ion intensity decreases and that there is very low wave intensity (in this frequency range) in the low energy electron precipitation region.

There is sometimes a very good correlation between intensifications in the precipitating electron fluxes and the observation of ion heating. One example is shown in Figure 11. The MATE and TESP electron data indicate anisotropic and variable electron fluxes, mainly below about 1 keV. The ion heating is confined to the same flux tubes. This need not be three different arcs that are crossed by Freja. Freja is moving more or less parallel to the auroral oval in the midnight sector and it is possible that Freja encounters the same structure several times if it has a wavy boundary.

On the other hand, the event shown in Figure 10 is better characterized as anti-correlated electron fluxes and ion precipitation accompanied by ion heating. That event seems to be best explained by assuming that lower hybrid waves are being generated by the anisotropy in the precipitating protons and that the cold ionospheric ions are being heated by the waves.

Figure 12 shows a perhaps more complicated event. There is an obvious anti-correlation between the electron precipitation and the regions with ion heating perpendicular to the magnetic field lines. The ion heating is seen in regions with downward field-aligned current (H. Lühr, personal communication) and increased wave intensities at low frequencies. Magnetic field spectra in the frequeny range 0-40 Hz are shown in the uppermost panel.

Several spaceraft have observed upward acceleration of electrons to keV energies at low altitudes. Upward electron beams are often associated with transverse heating of ions. Figure 13 shows a region with very low plasma density (10 cm -3) shortly before 0730 UT. Electrons (panel 3) are accelerated upward along the magnetic field to keV energies in this region. Large wave power at a few hundred Hz was observed together with an increase of the ion flux at keV energies in the 60-120° pitch angle range indicating perpendicular heating of the ions. A detailed description of this event can be found in Boehm et al. (1994).


We have presented Viking and Freja observations of macro- and meso-scale plasma density cavities and their relation to the heating and upward motion of ions. We draw the following main conclusions:

­ Outflow processes thrives in the low-density auroral cavity environment.

­ There is a clear relation between transverse ion energization as an expulsion process and the depth of the ionospheric plasma density inside the cavity - the stronger the energization, the lower the density inside the auroral plasma density cavity.

­ The plasma density inside the cavities may reach values below 10 cm -3 at an altitude of about 1700 km.

­ Transverse ion energization, as observed by Freja, occur over very broad regions in space and time.

­ There is a good correlation between LEF power spectral density and upflowing ion temperature. Ion conics give better correlation than ion beams. This means that it is a local acceleration.

­ An upward low energy ion bulk flow with a small amount of heating can sometimes be found inside regions with "inverted V" like electron precipitation. These ion bulk flow regions may contain strong plasma density depletion, thus suggesting that they represent the primary erosion associated with energetic ion beams at high altitudes.

­ Ionospheric plasma depletion does not result from a single physical process, but is rat her a consequence of many different ones.

The Viking observations often show electron distributions with enhanced fluxes at pitch angles close to the atmospheric loss cone. Some of the characteristics of electron conic distributions are:

­ They have a very narrow angular distribution of 10°.

­ They are seen together with ion beams and widened loss cones in the electron distribu tion, i.e., signatures of parallel electric field acceleration below the spacecraft.

­ There is an enhancement of the perpendicular temperature of the ion distribution at the time the electron conic is observed.

­ They occur most frequently in the dusk sector of the auroral oval.

­ They are generated in the 8.000 to 11.000 km altitude range

­ They are associated with upper hybrid waves and low frequency fluctuations.


The success of the Freja project is due to combined efforts of the Freja Scientific Team, the Swedish Space Corporation, FFV-Aerotech and MPE-Garching. The funding has been provided by the Swedish National Space Board, the Deutsche Agentur für Raumfahrtangelegenheiten (DARA), the Swedish Research Council (FRN), the Wallen berg foundation, Nordbanken and Sparbanken.


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