Air Force Geophysics Laboratory Magnetometer Research Articles

The ion cyclotron group delay for source regions near the plasmapause

Electromagnetic ion cyclotron waves are generated near the magnetic equator through an instability driven by the energetic ion temperature anisotropy. Many of these waves are observed on the ground as Pc 1 magnetic pulsations, which often exhibit a distinct wave packet structure (pearl pulsations). We use the warm dispersion relation to calculate the group delay of ion cyclotron waves from the equatorial source region near the plasmapause to the foot of the field line and compare the results to both cold theory and observations. Our results differ significantly from the cold plasma result for frequencies less than 0.85 fnof;cHe. In general we find that the presence of the ring current produces longer delays and greater wave dispersion. We model three particular ion cyclotron wave events each of which was observed simultaneously by DE 1 and the Air Force Geophysics Laboratory Magnetometer Network. Plasma data from three other instruments on DE 1 provide input parameters to the model. In two out of three cases our predicted delays agree with ground/satellite correlation delays. The third event does not agree as well with theory possibly due to the presence of a significantly greater O+ concentration during this event. The enhanced O+ concentration introduces an additional stop band below fnof;cHe along the propagation path which the linear theory is unable to model.

Ion cyclotron waves near L = 4.6: A ground‐satellite correlation study

We present results of the first ground‐satellite correlation study of ion cyclotron wave events inside geosynchronous orbit. We found several intervals during which waves at the same frequency were observed simultaneously by the DE 1 Goddard Space Flight Center Fluxgate Magnetometer and by the Air Force Geophysics Laboratory Magnetometer Network. This represents about 33% of the intervals during which waves were seen on the ground and DE 1 was in a favorable location (near the geomagnetic equator around L = 4.6) to observe the wave generation region. Three of seven simultaneous events showed a positive correlation when a detailed correlation analysis was performed. The broadband nature of the other events prevented any detailed correlation analysis. Peaks in the cross‐correlation function occurred at delays of 30–100 s and are interpreted as the group delay time of signals from space to the ground station. The small number of events seen in space during conjunction intervals results from the satellite's path missing the source region. When sources are missed by the satellite path, they either exist at higher or lower L shells and are ducted in the ionosphere to the network (L = 3) or they exist at the satellite L shell but are missed due to their small spatial extent in local time.

Simultaneous observation of a Pc 1 pulsation by the Air Force Geophysics Laboratory Magnetometer Network and GEOS 1

A Pc 1 event was observed simultaneously at the Air Force Geophysics Laboratory Magnetometer Network Newport station (L = 3, 1500–1600 MLT) and by GEOS 1 (L = 7, 1230–1310 MLT) on August 8–9, 1977, between 2325 and 0015 UT. Throughout the event the satellite and ground station were separated by at least 2.4 hours in local time and four L shells. At GEOS 1 the event had two frequency components, one just below and one just above the local He+ gyrofrequency. During the first part of the event these components are narrow‐banded. In the later stages of the event, the emissions observed in space are more broad‐banded. The first part of the event was observed on the ground; the lower‐frequency component was present, while the higher‐frequency component was detected only weakly. A comparison of wave spectra shows that the lower‐frequency component has the same frequency as a function of time in space and on the ground, but the temporal variations in the intensity and shape of the spectral peaks are different. Substorm onsets that occurred during the event are apparent as Pi 2 and PiB signatures in the ground‐based search coil data. These onsets may have affected the reception of the signal at Newport. We conclude that the source region was localized both in longitude and L shell, and on the basis of this conclusion we estimate the attenuation of the signal in the ionospheric duct to be 0.009–0.014 dB km−1.

A comparison of magnetic signatures and DMSP auroral images at substorm onset: Three case studies

We explore the relationship between mid‐latitude and synchronous orbit magnetic signatures and the location of the auroral surge at the onset of three isolated magnetospheric substorms. Mid‐latitude data come from the Air Force Geophysics Laboratory Magnetometer Network, synchronous orbit data from the satellites GOES 2 and 3, and auroral data from Defense Meteorological Satellite Program auroral images and auroral magnetograms. We find that the surge forms with its western edge ∼1 hour west of the longitude where the major axis of the mid‐latitude Pi 2 polarization ellipse is along magnetic north and where the D component perturbation of the magnetic bay is near zero. These observations are in qualitative agreement with a current wedge consisting of a localized upward current in the west and a longitudinally distributed current in the east. At synchronous orbit the longitude of the surge head appears to separate a region to the west where the magnetic field becomes more taillike from one to the east where the magnetic field becomes more dipolar.

Observations of field‐aligned currents, waves, and electric fields at substorm onset

Substorm onsets, identified by Pi 2 pulsations observed on the Air Force Geophysics Laboratory Magnetometer Network, are studied using magnetometer and electric field data from ISEE 1 as well as magnetometer data from the geosynchronous satellites GOES 2 and 3. The mid‐latitude magnetometer data provides the means of both timing and locating the substorm onset so that the spacecraft locations with respect to the substorm current systems are known. During two intervals, each containing several onsets or intensifications, ISEE 1 observed field‐aligned current signatures beginning simultaneously with the mid‐latitude Pi 2 pulsation. Close to the earth broadband bursts of wave noise were observed in the electric field data whenever field‐aligned currents were detected. One onset occurred when ISEE 1 and GOES 2 were on the same field line but in opposite hemispheres. During this onset ISEE 1 and GOES 2 saw magnetic signatures which appear to be due to conjugate field‐aligned currents flowing out of the western end of the westward auroral electrojets. The ISEE 1 signature is of a line current moving westward past the spacecraft. During the other interval, ISEE 1 was in the near‐tail region near the midnight meridian. Plasma data confirms that the plasma sheet thinned and subsequently expanded at onset. Electric field data shows that the plasma moved in the opposite direction to the plasma sheet boundary as the boundary expanded which implies that there must have been an abundant source of hot plasma present. The plasma motion was towards the center of the plasma sheet and earthwards and consisted of a series of pulses rather than a steady flow.

Longitudinal phase and polarization characteristics in mid‐latitude Pi 2 pulsations

Polarization and phase characteristics of mid‐latitude Pi 2 pulsations are studied using the Air Force Geophysics Laboratory Magnetometer Network. From the analysis of 55 Pi 2 events it is concluded that the sense of apparent phase velocity and the sense of polarization are related as follows: Westward phase velocity is associated with left‐handed polarization, while eastward phase velocity coincides with right‐handed polarization. Phase changes per degree longitude as determined by the D component are larger than those computed from the H component. These results are compared to a previous analysis of nighttime pulsations. The differences in apparent phase velocities are discussed with regard to a recent model for the Pi 2 longitudinal polarization pattern.

Mid‐latitude Pi 2 polarization pattern and synchronous orbit magnetic activity

Pi 2 pulsations are often used to indicate the start of the substorm expansion phase. In this report the relationship between the Pi 2 polarization pattern observed at a mid‐latitude longitudinal chain of stations and synchronous orbit magnetic activity is explored. Eighteen single‐ and multiple‐substorm‐onset intervals, consisting of 55 Pi 2's, are examined using data from the Air Force Geophysics Laboratory Magnetometer Network and the synchronous orbit satellites GOES 2 and 3. Magnetic fluctuations at synchronous orbit are found to be enhanced when the satellite is near the Pi 2 pattern center, defined as the longitude where the major axis of the polarization ellipse is along the H axis. The pattern center is also observed to separate regions of dipolarization and more taillike magnetic signatures. East of the pattern center the field inclination becomes larger, i.e., more dipolelike, while west of the center the field inclination becomes smaller, i.e., more taillike. Although regions of substorm activation, as determined by the Pi 2 pattern, move between onsets during multiple‐onset events, no consistent azimuthal motion was found.

Mid‐latitude Pi 2 pulsations, geosynchronous substorm onset signatures and auroral zone currents on March 22, 1979: CDAW 6

We relate the mid‐latitude Pi 2 pulsation signatures observed using the Air Force Geophysics Laboratory Magnetometer Network during the March 22, 1979, Sixth Coordinated Data Analysis Workshop interval to the magnetic variations observed at geosynchronous orbit and to ionospheric current distributions derived from high latitude magnetometer data and an ionospheric conductivity model. We are able to show that at least three separate substorm onsets or intensifications occurred around 1100 UT, specifically at 1053:50, 1104:10 and 1110:30 UT. We contrast an earlier onset which occurred at 0614 UT during a geomagnetically quiet interval to these more disturbed events. In both cases the Pi 2 polarization pattern locates the region of strong westward electrojet. Changes in the Pi 2 polarization during a single event appear to accompany small shifts in the electrojet position. These events do not obey the Pi 2 period‐electrojet latitude relationship obtained by Kuwashima and Saito (1981).