Shing F. Fung

Evidence of high-latitude reconnecting during northward IMF: Hawkeye observations

Reconnection is accepted as an important process for driving the solar wind/magnetospheric interaction although it is not fully understood. In particular, reconnection for northward interplanetary magnetic field (IMF) at high-latitudes tailward of the cusp, has received little attention in comparison with equatorial reconnection for southward IMF. Using Hawkeye data we present the first direct observations of reconnection at the high-latitude magnetopause (75°) during northward IMF in the form of sunward flowing protons. This flow is nearly field aligned, approximately Alfvenic, and roughly obeys tangential momentum balance. The magnetic field shear is large at the magnetopause and there is a non-zero BN component suggesting the existence of a rotational discontinuity and reconnection. The Hawkeye observations support several recent simulations at least qualitatively in terms of flow directions expected for high-latitude reconnection during northward IMF.

On the origin of whistler mode radiation in the plasmasphere

[1] The origin of whistler mode radiation in the plasmasphere is examined from 3 years of plasma wave observations from the Dynamics Explorer and the Imager for Magnetopauseto-Aurora Global Exploration spacecraft. These data are used to construct plasma wave intensity maps of whistler mode radiation in the plasmasphere. The highest average intensities of the radiation in the wave maps show source locations and/or sites of wave amplification. Each type of wave is classified on the basis of its magnetic latitude and longitude rather than any spectral feature. Equatorial electromagnetic (EM) emissions (30–330 Hz), plasmaspheric hiss (330 Hz to 3.3 kHz), chorus (2–6 kHz), and VLF transmitters (10–50 kHz) are the main types of waves that are clearly delineated in the plasma wave maps. Observations of the equatorial EM emissions show that the most intense region is on or near the magnetic equator in the afternoon sector and that during times of negative Bz (interplanetary magnetic field) the maximum intensity moves from L values of 3 to <2. These observations are consistent with the origin of this emission being particle-wave interactions in or near the magnetic equator. Plasmaspheric hiss shows high intensity at high latitudes and low altitudes (L shells from 2 to 4) and in the magnetic equator with L values from 2 to 3 in the early afternoon sector. The longitudinal distribution of the hiss intensity (excluding the enhancement at the equator) is similar to the distribution of lightning: stronger over continents than over the ocean, stronger in the summer than in the winter, and stronger on the dayside than on the nightside. These observations strongly support lightning as the dominant source for plasmaspheric hiss, which, through particle-wave interactions, maintains the slot region in the radiation belts. The enhancement of hiss at the magnetic equator is consistent with particle-wave interactions. The chorus emissions are most intense on the morningside as previously reported. At frequencies from 10 to 50 kHz, VLF transmitters dominate the spectrum. The maximum intensity of the VLF transmitters is in the late evening or early morning with enhancements all along L shells from 1.8 to 3.

Staircase ion signature in the polar cusp - A case study

On 15 October 1981 Dynamics Explorer 2 crossed the polar cusp at 1015 MLT and observed three distinct ion populations as it was moving poleward. These three populations had peak-flux energy around 2.7 keV, 850 eV and 360 eV. At the time of observation, the IMF was southward. The first step coincided with a rotation of the flow from westward to poleward and then eastward. The second and third steps showed a flow directed principally poleward. Furthermore, the magnetic and electric perturbations in the first step are well fitted by an elongated flux tube footprint model. These results suggest that three consecutive Flux Transfer Events (FTEs) have injected solar wind plasma into the ionosphere forming the polar cusp. The individual FTE signatures can only be identified by the jumps in the precipitation pattern. The newest reconnected FTE footprint was crossed near the edge of the event while the two oldest ones were crossed around the event center. The small latitudinal size of these FTE footprints ([approximately]40 km) and their short recurrence rate (3,6 min) is consistent with an intermittent reconnection taking place at the subsolar point on a short time scale. 21 refs., 4 figs.

Observation of magnetospheric relativistic electrons accelerated by Pc‐5 ULF waves

[1] Magnetic field, electric field and energetic electron measurements from CRRES, GOES-6 and -7 satellites are used to investigate the production of up to 2 MeV electron fluxes through a storm sudden commencement (SSC) event on August 27, 1991. Strong Pc-5 oscillations, whose electric field is mainly along the radial direction, and up to an order-of-magnitude enhancement in relativistic electron fluxes occurred simultaneously in a two-hour period. The enhanced electron fluxes are found to have a pancake-like pitch angle distribution, which is consistent with the fluxes being accelerated near the equatorial plane. The electron acceleration is shown to result from drift-resonant interactions with the toroidal-mode Pc-5 ULF waves having wave frequencies three times the electron drift frequency. In view of the L-variation of ULF wave frequencies, electron acceleration to relativistic energies becomes more effective in the geosynchronous region.

High-altitude cusp positions sampled by the Hawkeye satellite

Abstract The Hawkeye satellite, with its almost 90° inclination and 21 RE apogee, provided the most extensive sampling to date of the exterior cusp as well as the high-latitude magnetopause. Using a combination of plasma, magnetic field, and plasma wave observations sampled between 1974 and 1978, we have identified 211 crossings of the exterior cusp, of which 175 have solar wind pressure and 136 have simultaneous IMP-8 interplanetary magnetic field (IMF) data available. This ensemble of cuts through the exterior cusp region was then statistically analyzed to determine possible variations of average cusp position with changes in IMF orientation or solar wind pressure. Our analyses show that the exterior cusp region (> 4 RE) is best ordered in Solar Magnetic (SM) coordinates. It has an average magnetic latitude of about 62° and average invariant latitude near 81°. There is substantial inward and outward radial distance change with changing solar wind pressure. The Hawkeye exterior cusp samples show weak or no dependence on IMF orientation, at least for changes on time scales longer than about one hour. Effects due to shorter time-scale variations, like those most often seen at low-altitudes, would not be apparent in our statistical results. Comparison of the Hawkeye samples of the exterior cusp with low-latitude magnetopause observations suggests that the frontside magnetosphere is asymmetric with more stretching in the polar direction.

Time correlation of low-altitude relativistic trapped electron fluxes with solar wind speeds

We present the results from a study of time correlation between the low-altitude relativistic trapped electron fluxes and the solar wind speeds. Our trapped electron observations in the energy range of 0.19–3.2 MeV were obtained by the OHZORA spacecraft in an altitude range of 350–850 km, near a solar minimum period (1984–87). The solar wind data with a 5-minute time resolution were obtained from IMP-8 observations. Linear correlation analyses between the two data sets have been performed for relative time lags varying from 12 minutes to 60 days. The 2.5–day, 13-day, 27-day and 54-day correlation peaks previously reported for energetic electrons near geosynchronous orbits are clearly seen in our results. However, the use of higher time resolution solar wind data than in previous studies allows correlation analyses to be performed at shorter time lags. We report here that correlation at shorter time lags (<10 hrs) exists and that while such correlation is stronger than those observed at longer lag times, it can not be entirely attributed to storm or substorm injections. The correlation is also found to decrease with drift shell magnetic equatorial radii, rt. In addition, local-time and radial variations in the responses of different drift shells to solar wind speed enhancements indicate that the energetic electron population enters the inner magnetosphere predominantly through the midnight sector.

Observations of the latitudinal structure of plasmaspheric convection plumes by IMAGE‐RPI and EUV

Received 22 May 2002; revised 16 April 2003; accepted 2 May 2003; published 15 August 2003. [1] Recent IMAGE Extreme Ultraviolet Imager (EUV) observations showed the first global images of plasmaspheric convection plumes, which have been interpreted as the plasmaspheric tails predicted theoretically 3 decades earlier. Using observations by the IMAGE Radio Plasma Imager (RPI), we show that these convection plumes have large latitudinal extent. These results complement those recently made by others in correlating IMAGE EUV data with measurements of total electron content in the ionosphere. By correlating in situ RPI density measurements with global plasmaspheric EUV images, we have shown that apparently detached plasma structures, as appear in RPI dynamic spectrograms, are in many cases plasmaspheric convection plumes. The temporal separation between the RPI and EUVobservations help constrain the interpretation of one data set in the context of the other, thereby enabling an examination of the threedimensional plasma density structures outside the core plasmasphere. The data sets are mutually reinforcing because the data are collected within a few hours of one another. We used the EUV data to provide unambiguous identification of density enhancements in the region outside the plasmasphere and used the RPI data to obtain accurate number densities and extend information from the EUV data set by measuring densities below the EUV sensitivity threshold. INDEX TERMS: 2768 Magnetospheric Physics: Plasmasphere; 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2730 Magnetospheric Physics: Magnetosphere—inner; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: plasmasphere, plasmapause, inner magnetosphere, convection plumes, RPI, EUV

Structure within Jovian hectometric radiation

Observations of Jovian hectometric radio emission (HOM) by the Voyager planetary radio astronomy (PRA) experiment at frequencies from 300 kHz to 1.3 MHz indicate persistent dynamic spectral features that had not been previously studied. The features of interest appear as “lanes” of decreased emission intensity within the otherwise persistent HOM. The lanes are apparent in intensity and occurrence probability spectrograms of frequency versus Jovian System III (1965) longitude. In the investigation of the morphology of these features, we use inbound and outbound Voyager 2 data at Jupiter to show that the lane occurrence and characteristics do not depend on local time over the range sampled. Occurrence probability spectrograms of frequency versus magnetic latitude are created from the portion of the data when the spacecraft was between 0° and +10° magnetic latitude. These spectrograms represent both the inbound and outbound passes and are quite similar despite the different longitude ranges. A simple extension of decametric (DAM) arc features into the HOM wavelength does not account for all the lane features, giving further evidence that HOM is an independent emission component. Polarization signatures for the data show that the polarization is predominantly right-hand circular and that it does not reverse across the lanes, suggesting the emission is from the same hemisphere. In addition, we investigate possible effects due to solar wind variations and find that the occurrence of the lanes appears to be independent of times of low and high solar wind densities. The intensity of the HOM emission on either side of the lanes is comparable, implying that the lane is probably not a result of a gap between fundamental and second harmonic emission regions. We present these data and analyses as a morphological study to establish that the lane features are an important part of the HOM emission and should be considered in HOM emission models. At this time, no theory of the source of the lanes explains all the observed features.

Latitudinal structure within Jovian hectometric radiation

Jovian hectometric radio emission (HOM: 300–3000 kHz) has a number of persistent structural features associated with it as observed by the Voyager 1, Voyager 2, Ulysses, and Galileo spacecraft for specific jovigraphic latitudes (−4° to +7.1°) and local times (0.3 to 10.5 hours). Most notable are the presence of HOM emission between 270° and 120° central meridian longitude (CML) and the region of reduced emission intensity (a “gap”) between 120° and 270°. We displayed the Ulysses and Galileo data using time-frequency occurrence probability spectrograms and show that the observed HOM emission features are nearly identical to those observed by the Voyager spacecraft. This implies that the HOM structure is long-lived and fixed in its longitudinal position within the Jovian magnetosphere. HOM structure depends on small changes in the observers jovigraphic latitude, so the different jovigraphic latitudes of the spacecraft were used to probe the HOM beam structure. Prom this analysis we found that the CML width of the main HOM gap is directly correlated to the latitude of the spacecraft. We conclude that the latitudinal thickness of the HOM beam is about 12°, extending from −5° to +7° magnetic latitude.

Radio wave active Doppler imaging of space plasma structures: Arrival angle, wave polarization, and Faraday rotation measurements with the radio plasma imager

Radio sounding in the magnetosphere by the radio plasma imager on the IMAGE spacecraft will determine the dimensions and shape of the cavity between the magnetopause and the plasmapause. Omnidirectional transmission of pulsed radio signals results in echoes arriving from many directions. Quadrature sampling and Doppler analysis of the signals received on three orthogonal antennas will make it possible to determine the angles of arrival of the echoes, their polarization ellipses, and the Faraday rotation. Decomposition of the echo signals into the two characteristic waves is used to identify the O- and X-wave components.