*************************** ** THE GEM MESSENGER ** *************************** Volume 9, Number 25 July 8, 1999 ---------------------------------------------------------------------------- 1999 Snowmass Workshop Report - Special Sessions on Groundbased Magnetometry ---------------------------------------------------------------------------- From: Chris Russell (ctrussel at igpp.ucla.edu) Coordinator: C. T. Russell At the 1999 Snowmass meeting a special effort was made to convene a set of special sessions of importance to the on-going campaigns (Inner Magnetosphere/ Storms and Magnetotail/Substorms), but also of interest to the ground-based magnetometer community that has much to provide to the GEM program but had not been as fully exploited as it could have been in past meetings. Four special sessions were arranged: two under the inner magnetosphere storms campaign; one under the tail substorm campaign and one technical session crossing the campaigns boundaries. The two inner magnetosphere/storm campaigns covered magnetic pulsation sounding of the magnetospheric density and SI propagation through the magnetosphere. The tail/substorm session covered the propagation of Pi2 pulsations. The technical session reviewed the status and plans of the ground-based magnetometry community. Reports on each of these sessions follow below. I. Inner Magnetosphere/Storms ============================= I.1. Magnetic Pulsation Sounding of the Magnetospheric Density The Newcastle magnetometer group under the leadership of Brian Fraser has pioneered the determination of the equatorial mass density using resonating magnetic field lines at Pc3 frequencies. If one can determine the location of a resonating field line, one can solve for the equatorial mass density, knowing the "Length of the line" and the frequency of the resonance. The working group discussed techniques and the application of these techniques to the magnetosphere. The most popular technique uses the phase difference between the H-component oscillations at two adjacent stations in a meridian chain. Peter Chi showed that this phase difference depended on both the distance between stations and the state of the ionosphere. This technique requires precise timing. An alternate technique was discussed by C. T. Russell in which the H-component power ratio between adjacent stations was examined instead of the phase. This approach identifies two resonating field lines (one over each station and does not require precise timing). Brian Fraser and Ian Mann demonstrated that the plasma density sounding could be performed from L=1.5 to over L=7. Most importantly the location of the plasmapause could be resolved. Peter Chi showed that a large magnetic storm depleted the density in the plasmasphere inside L=2. And the refilling of the plasmasphere could be followed over the next few days. The session revealed that Pc3 resonance sounding had come of age as an effective tool for monitoring the magnetospheric plasma. However, some new stations are needed especially bridging the gap between L=3 and 4 and several longitudes need coverage, not just one. I.2. Sudden Impulse Propagation through the Magnetosphere One of the earliest problems in magnetospheric physics is how the magnetosphere is compressed when the pressure in the solar wind suddenly increases. The change in the boundary condition on the magnetosphere is simple. The magnetopause moves closer to the surface of the Earth. BUT - the geometry is complicated by the dipolar magnetic field - the electrical conductivity of the ionosphere is a tensor - the mass in the magnetosphere must move when the boundaries move - MHD waves propagate in three different modes The working group concentrated on two different areas: - first arrival timing and phenomenology (the first minute) - plasma circulation and currents induced by the compression (the first hour) The first arrival is a twist in the field at the leading edge of the compression. On the afternoon side this twist leads to a "preliminary reverse impulse" (PRI). On the morning side the twist gives a positive H perturbation that merges with the following compression. Prior to the advent of modern magnetometers with high temporal accuracy and rapid sampling it was believed that the PRI moved at the speed of light. Present data do not confirm this. D. H. Lee showed how these first arrival times could be predicted with a simple linear model. The post-compression circulation has been addressed in two ways: - AMIE modeling using observed current patterns - MHD modeling from solar wind conditions across the shock as it approaches the Earth. (The results of both techniques are in substantial agreement with each other and with observations). Both approaches show that the magnetospheric response is dependent on IMF conditions when the stock hits the magnetosphere. The session demonstrated that the tools and observations needed to understand SI effects are now available and the outstanding issues in this area will soon be resolved. II. Tail/Substorm ================= Pi2 Propagation Summary Pi2 waves are very helpful in substorm studies because they can be detected far from their source BUT - substorms can occur without observations of Pi2s - Pi2s can occur without the occurrence of substorms. Thus we need to understand the factors that control the source and propagation of these waves in the magnetosphere. This session focussed on propagation effects. Propagation through the magnetosphere adds a transit time delay to the signal and alters the waveform of the signal as it passes through resonant and 'virtually' resonant regions. In particular the density in the plasmasphere significantly delays the signals, causes "anomalously" late arrivals and creates a resonant cavity. * We need to understand better both theoretically and observationally - the propagation delays in order to time substorm onsets - the alteration of the signal wave form * We need to devise better means of deriving the first arrived times of pi2 waves from our observations The workshop caught this topic at a moment where progress is being made rapidly but with more work yet to be done. III. Technical Session on Ground-based Magnetometry =================================================== Ground-based magnetic records have long played a crucial role in understanding the behavior of the magnetosphere and ionosphere. Our understanding of the magnetosphere has reached a stage where we need timing accuracies of 1 second or better and often sub nT precision. Moreover, it is no longer enough to have single sites, or even chains, but rather two dimensional arrays are needed. Fortunately, it is possible to build such precision instruments relatively inexpensively, and many groups have done so. Yet some regions are not adequately covered. In this session we reviewed the status of operating networks, their data availability, their sampling frequency and future plans. Arrays discussed included the MACGS array, the Danish Greenland array, the Magic and Antarctic arrays, the CPMN array, SAMNET, the Newcastle array, the MEASURE array, the SMALL array, the Chimag array, the Mexican array, the USGS array and proposals for floating magnetometers on the ice cap and a new array in South America. Material was submitted to the session by several who could not attend and reported to the attendees by the chairman. This material included discussions of the IMAGE array, the French magnetometer stations, Lucent Technologies sites and the Italian stations. It was clear from the scientific presentations that much new was being learned from the ground-based measurements but that additional sites and renovations were needed to fully exploit the techniques now available to probe the ionosphere and magnetosphere from the ground. For example Brian Fraser reported that resonance sounding of the plasmaspheric density could be performed as low as an L value of 1.3 and Ian Mann showed that the same technique could be used as high as L=7. Brian Fraser noted that he had been able to look for the last closed field line with stations near L=9. Peter Chi showed that you could also determine the Q of the ionosphere with these data. The various presentations on SI and Pi2 propagation were equally as revealing. It was clear that velocities of the disturbances could be measured with timing resolution of 1s or better. It was very obvious that a time resolution of 1 second with accurate timing was now essential for magnetospheric studies and many groups announced their plans to do so. It was also obvious that meridian chains useful for resonance sounding were not yet adequate. IMAGE and the Alaskan chains might cover the region from L=4 to 7 and MEASURE might cover L=2 to 3 but there was a gap between L=3 and 4 and we needed full L value coverage at several longitudes before we could adequately cover the filling and emptying of the magnetosphere. Other recommendations resulting from the session included continued improvements in the numerical models of wave propagation, increasing the sample rate to 2 Hz to include the study of Pi1 pulsations, filling in the Russian sector and the undertaking of conjugate point studies. The proposal to study the IMF through a longitudinal chain of stations near 80 deg was also presented. This technique holds the promise of providing the IMF By and Bz components averaged over a period of 30 minutes. Depending on the rapidity of changes that affect substorms this may or may not assist in substorm studies. In summary, the session was both well attended and very interactive. The material gathered from the various presenters will be assembled and posted on the web to facilitate collaboration and data exchange. We recommend that a similar coordinated approach to the organization of magnetometer-specific sessions and the discussion of technical issues be followed at the next GEM meeting. +-------------------------------------------------------------------------+ |To add name to the mailing list or for a message to the GEM community | | please contact: editor at igpp.ucla.edu | | | |URL of GEM Home Page: http://www-ssc.igpp.ucla.edu/gem/Welcome.html | |Please update your e-mail address. | +-------------------------------------------------------------------------+