c Annales Geophysicae (2002) 20: 1711–1724 European Geosciences Union 2002 Annales Geophysicae Polar observations of electron density distribution in the Earth’s magnetosphere. 1. Statistical results H. Laakso1, R. Pfaff2, and P. Janhunen3 1ESA Space Science Department, Noordwijk, Netherlands 2NASA Goddard Space Flight Center, Code 696, Greenbelt, MD, USA 3Finnish Meteorological Institute, Geophysics Research, Helsinki, Sweden Received: 3 September 2001 – Revised: 3 July 2002 – Accepted: 10 July 2002 Abstract. Forty-five months of continuous spacecraft poten- Key words. Magnetospheric physics (magnetospheric con- tial measurements from the Polar satellite are used to study figuration and dynamics; plasmasphere; polar cap phenom- the average electron density in the magnetosphere and its de- ena) pendence on geomagnetic activity and season. These mea- surements offer a straightforward, passive method for mon- itoring the total electron density in the magnetosphere, with high time resolution and a density range that covers many 1 Introduction orders of magnitude. Within its polar orbit with geocentric perigee and apogee of 1.8 and 9.0 RE, respectively, Polar en- The total electron number density is a key parameter needed counters a number of key plasma regions of the magneto- for both characterizing and understanding the structure and sphere, such as the polar cap, cusp, plasmapause, and auroral dynamics of the Earth’s magnetosphere. The spatial varia- zone that are clearly identified in the statistical averages pre- tion of the electron density and its temporal evolution dur- sented here. The polar cap density behaves quite systemati- ing different geomagnetic conditions and seasons reveal nu- cally with season. At low distance (∼2 RE), the density is an merous fundamental processes concerning the Earth’s space order of magnitude higher in summer than in winter; at high environment and how it responds to energy and momentum distance (>4 RE), the variation is somewhat smaller. Along transfer, both in response to the solar wind, as well as to inter- a magnetic field line the density declines between these two nal sources, such as the ionosphere (e.g. Moore and Delcourt, altitudes by a factor of 10–20 in winter and by a factor of 1995). Although the electron density in the magnetosphere 200–1000 in summer. A likely explanation for the large gra- is known in a general sense from case studies, large-scale, dient in the summer is a high density of heavy ions that are systematic studies of the electron density distribution in the gravitationally bound in the low-altitude polar cap. The geo- magnetosphere have been performed only a few times in the magnetic effects are also significant in the polar cap, with the past (e.g. Escoubet et al., 1997; Laakso et al., 1997; John- average density being an order of magnitude larger for high son et al., 2001). Consequently, the models of the magne- Kp; for an individual case, the polar cap density may increase tospheric plasma density are much poorer than those for the even more dramatically. The plasma density in the cusp is magnetic field. The latter have been under development for controlled primarily by the solar wind variables, but never- many decades (e.g. Tsyganenko and Stern, 1996) and have theless, they can be characterized to some extent in terms of benefited from the fact that almost every scientific satellite in the Kp index. We also investigate the local time variation of the magnetosphere has carried a magnetometer. Plasma den- the average density at the geosynchronous distance that ap- sity measurements are more difficult to obtain in a systematic pears to be in accordance with previous geostationary obser- sense from a single instrument, since the plasma density and vations. The average density decreases with increasing Kp temperature might typically vary over several orders of mag- at all MLT sectors, except at 14–17 MLT, where the average nitude. density remains constant. At all MLT sectors the range of the There are a number of techniques for measuring the total density varies by more than 3 orders of magnitude, since the plasma number density, but each technique has its own limi- geostationary orbit may cut through different plasma regions, tations. Spacecraft charging seriously limits the direct detec- such as the plasma sheet, trough, and plasmasphere. tion of low-energy charged particles and must be taken into account for a variety of instruments. Furthermore, photoelec- Correspondence to: H. Laakso ([email protected]) trons and secondary electrons can cause a significant increase 1712 H. Laakso et al.: Polar observations of electron density distribution in the Earth’s magnetosphere in the density around the spacecraft which degrades the qual- ity of low-energy electron density measurements. Plasma magnetosheath wave cutoffs and resonances can provide excellent measure- bow shock ments of the plasma density if such waves are detectable, which is not always the case in tenuous plasmas. Also, some- times, these waves do not represent the local plasma where the satellite is located. Electric field double probe instru- lobe magnetopausepolar cap ments (EFI) provide measurements of the potential differ- cusp ence between a sensor and the satellite body, which depend 7 9 5 **** plasma- on the total electron density, as we discuss below. * **11 pause * * In addition to electric field measurements, electric field 3* *13 * double probes typically monitor the potential difference * 15* 1 plasma sheet (1V ) between each sensor and the satellite (Mozer et al., * 17 * * * 1978; Pedersen et al., 1978). In the case of the Polar satel- lite, the probe potential is kept artificially near the local plasma- plasma potential by means of a bias current. The measured sphere potential difference, therefore, is approximately the space- cusp craft potential (with respect to the plasma potential) which polar cap lobe is, on the other hand, proportional to the electron density and only weakly dependent on the electron temperature (Laakso and Pedersen, 1998). Therefore, 1V measurements offer a straightforward, passive method for monitoring the variation of the total electron density in the magnetosphere with a high time resolution (e.g. less than 1 msec) and a range that cov- ers many orders of magnitude. The accuracy of this tech- Fig. 1. Schematic drawing of the Earth’s magnetosphere in the nique remains valid even for densities well below 1 cm−3, noon-midnight meridian. A solid line presents a Polar trajectory; as shown by numerous in situ measurements in the magne- asterisks give the spacecraft’s position at one-hour intervals. tosphere (e.g. Pedersen, 1995, and references therein). This technique has been applied in several studies of electron den- sity variations in the magnetosphere (e.g. Escoubet et al., perigee (initially over the Southern Hemisphere), and an or- 1997; Laakso et al., 1997; Johnson et al., 2001). bital period of about 18 h. The orbital plane rotates about the Earth with respect to the Sun in one year so that all local This paper deals with the distribution of the average elec- times are covered in a 6-month period. Figure 1 presents tron density in the magnetosphere, using 45 months of satel- a schematic drawing of the magnetosphere at the noon- lite potential data from the EFI experiment on the Polar satel- midnight meridian and a Polar orbit, where asterisks mark lite. These data cover 107 data points (one-minute averages the position of the satellite at one hour intervals. are used here) distributed between 2 and 9 RE geocentric dis- tances along Polar’s polar orbit. The plasma regions that di- rectly emerge from the statistical pictures are the cusp, polar 2.1 Derivation of total electron density cap, and plasmasphere. The data set allows us to investi- gate the variation of the electron density in these and many The Polar EFI experiment (Harvey et al., 1995) consists of other regions. The present study contains a detailed analysis three pairs of double probe antennas oriented perpendicular of the polar cap density variation with season, altitude, and to each other. The potential difference between each probe geomagnetic activity. We also investigate how the average (probe potential = Vp) and the satellite (satellite potential = electron density varies at L = 6.6, that is, at the geosyn- Vs) is chronous distance, as a function of MLT and geomagnetic activity. In the second part of this study, Laakso et al. (2002) 1Vps = Vp − Vs, (1) present high-resolution data from individual orbits and ex- amine in more detail some specific magnetospheric regions, where p corresponds to the sensor number, and in this case, such as the cusp, plasmapause, trough, and polar cap. p = 1–6. These are sampled at a high-time resolution (10 or 20 Hz, typically), which allows us to follow rapid elec- tron density variations along the Polar trajectory. In burst 2 Instrumentation mode the sampling rate can be up to 8000 samples per sec- ond, in which case very fast density variations can be moni- We use measurements gathered by the electric field instru- tored, which are commonly observed, for instance, on auroral ment (EFI) on the Polar satellite. Polar was launched on field lines (Laakso et al., 1997). ◦ 24 February 1996 into a 90 inclination orbit with a 9 RE To minimize the contributions of the DC electric fields on apogee (initially over the Northern Hemisphere), a 1.8 RE the spacecraft potential measurements, the measurements of H. Laakso et al.: Polar observations of electron density distribution in the Earth’s magnetosphere 1713 PP PP PP PP PS DAY cusp northern polar cap AUR PSSH PS 0 200 -5 6 -10 1 density (cm -15 0.4 (volts) 12s –3 V -20 0.2 ) ∆ PS - plasmasphere PP - plasmapause -25 DAY - dayside magnetosphere AUR - northern auroral zone Polar EFI SH - southern polar cap & auroral zone April 29, 1996 -30 0.1 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 UT 11:20 11:53 14:36 19:30 21:17 22:08 22:44 23:10 11:32 MLT 7.3 39.7 209.3 131.2 38.8 16.6 8.2 4.1 2.4 L shell 68.3 80.9 86 85 80.8 75.8 69.6 60.4 49.8 Inv Lat (˚) Fig.