Supplementary material to “Formation, Dynamics, and Impact of Plasmaspheric Plumes”

Jerry Goldstein, Southwest Research Institute, San Antonio, Texas; Joseph Borovsky, Los Alamos National Laboratory, Los Alamos, New Mexico; John Foster, MIT Haystack Observatory, Westford, Massachusetts; Donald Carpenter, Stanford University, Stanford, California

Citation:
Goldstein, J., J. Borovsky, J. Foster, and D. Carpenter (2007), Formation, dynamics, and impact of plasmaspheric plumes, Eos Trans. AGU, 88(23), 247. [Full Article (pdf)]

Plasmaspheric plumes result from erosion of the plasmasphere. The Institute of Geophysics and Planetary Physics (IGPP) Workshop on Plasmaspheric Drainage Plumes was convened in Taos, New Mexico to examine outstanding questions about the formation and dynamics of plumes, and the impact of plumes on the near-Earth space environment (geospace). Some of these outstanding issues are discussed herein and a call is made to researchers to participate in the analysis of several interesting geomagnetic events that were selected during the workshop.

Plasmaspheric Erosion and Plume Formation

The plasmasphere is an upward extension of the Earth’s ionosphere. It is a region of relatively dense, cool plasma (100 to 10,000 e- cm-3 and of the order of 104 K) surrounding the Earth. The plasmasphere is formed by ionospheric outflow, in which plasma diffuses into space and loads geomagnetic field lines with a mixture of protons, helium, and oxygen. During extended quiet periods outflow can expand the plasmasphere to beyond geostationary orbit (6.6 Earth radii or RE).

In geospace, quiet conditions are inevitably interrupted by disturbances such as geomagnetic storms and substorms, all triggered by the solar wind (the Sun’s outward-streaming atmosphere) that confines Earth’s magnetic field into a region called the magnetosphere. The solar-wind-magnetosphere interaction drives convection, a global plasma circulation involving a sunward flow in the inner magnetosphere. Caught up in this sunward convection, the outer plasmasphere is stripped away, a process known as plasmaspheric erosion. Erosion results in a global reduction of the size of the plasmasphere. On the dayside, it produces plumes of eroded plasma that extend sunward in the prevailing convection stream. In the aftermath of erosion events, plumes rotate and wrap around the main plasmasphere, and ionospheric outflow gradually replenishes the eroded plasma.

The theoretical prediction of plasmaspheric plumes by Grebowsky [1970] was bolstered by decades of research [for example, Lemaire and Gringauz, 1998, and references therein]. More recently, the existence of plumes was proven conclusively with extreme ultraviolet (EUV) plasmasphere images [Sandel et al., 2001]. Although the convective origin of plumes is now well established [Goldstein and Sandel, 2005, and references therein], many questions remain concerning the details of plume formation and dynamics. Also of increasing interest are the ways in which plumes exert an influence on the rest of Earth’s space environment.

Mass Transport and the Fate of Plasmaspheric Plumes

When the interplanetary magnetic field (IMF) and the Earth’s intrinsic field undergo reconnection (a magnetic reconfiguration that opens magnetic flux tubes up to the IMF) solar wind momentum is strongly coupled to the magnetosphere, driving the enhanced convection that creates plumes.

Although it is generally accepted that convection carries plume plasma to the dayside magnetopause (the boundary of the magnetosphere), what is the fate of plumes that encounter the reconnection region? Data and theory presented at the workshop implied that plume plasma may be carried along with newly-reconnected field lines, dragged over the Earth’s magnetic polar cap and possibly onto the nightside. However, the evolution of plasmaspheric material on flux tubes that have been opened to the IMF by reconnection is unknown. Is plume plasma on open field lines drained into the solar wind? Is it energized at the reconnection site, or by intermingling with the “magnetosheath” of shocked solar wind plasma outside the magnetopause?

How Connected are the Plasmasphere and Ionosphere?

At the workshop many suggested that steep density gradients at the plasmapause boundary layer map along magnetic field lines to a similar density gradient in the ionosphere. Plumes have been demonstrated to map to corresponding storm-enhanced density (SED) “tongues” in the ionosphere [Foster et al., 2002], especially while erosion is actively shaping the plasmasphere, but often also during the several hours after erosion has ceased, when “fossil plumes” rotate eastward and become wrapped around the recovery-phase plasmasphere.

These results do signify an intimate relationship between the ionosphere and plasmasphere, given their shared geomagnetic field lines and plasma. Nonetheless, questions persist concerning this relationship. Does convection erode dayside plasma from the ionosphere? If so, is it true that after very large storms ionospheric outflow is reduced, and recovery of the outer plasmasphere is delayed? Can upflow of high-latitude ionospheric plasma affect the density of plumes if they are dragged over the polar cap? What are the precise conditions required for a plume to have a corresponding SED tongue feature?

Plumes Over the American Sector

The workshop featured recent efforts to continue the investigation of ionospheric plume signatures begun by Su et al. [2000] and Foster et al. [2002]. The newer results suggest something unexpected: a longitudinal or geographic dependence of the density of plasmaspheric plasma. Plasmaspheric density apparently exhibits an annual density variation (with a December maximum) whose amplitude is greatest at about 75 degrees west geographic longitude (Eastern United States and Canada). It was also suggested at the workshop that the frequency of observation of ionospheric signatures of plumes is highest in the eastern American longitude sector, and that plumes in this sector have higher density than elsewhere. If this geographic-longitude dependence is verified with magnetospheric spacecraft, it not only must be explained; it presents a new set of conditions on the erosion of the plasmasphere. Is there a December maximum in plume density? Is there a global maximum in plume density when plumes are drawn from regions near the 75°W meridian? Is there evidence of reduced magnetospheric shielding near 75°W longitude, especially in the austral winter?

Space Environmental Impact of Plumes

The presence of dense plasmaspheric plasma in the outer magnetosphere can exert a strong (though indirect) influence on the dynamics of hotter populations such as the ring current (temperatures ranging from 10 to 100 million K) and Van Allen radiation belts (greater than 1 billion K). Intermingling of ring current ions with cold dense plume plasma favors the growth of electromagnetic ion cyclotron (EMIC) waves that in turn can scatter ring current ions and outer Van Allen belt electrons into the atmosphere. While the workshop made it clear that plume EMIC waves control critical aspects of hot particle dynamics, our community’s knowledge of EMIC wave growth inside plumes is not yet sufficient to reliably predict those dynamics. We need to characterize the general spatial, temporal, and activity-level dependences of the amplitude and frequency of EMIC waves in plumes. In theoretical models [e.g., Khazanov et al., 2006] EMIC waves grow at the steep density edges of plumes, but observations show EMIC waves throughout the plumes’ interiors. Do EMIC waves grow in association with the many density fluctuations inside plumes?

Plasmaspheric Oxygen

Although protons usually dominate the magnetosphere, impulsive disturbances (such as substorms) can increase the fraction of oxygen ions [Fok et al., 2006]. A hot topic at the workshop was mounting evidence by Fraser et al. [2005] (and others) of a torus of enhanced O+ in the outer plasmasphere whose characteristics require exploration. What is the dynamic dependence of the spatial relationship between the O+ torus and the proton plasmasphere? Does the O+ torus originate from outflow? Is it a plasmaspheric or auroral population? Answering these questions is a high priority because O+ ion enrichment adjacent to drainage plumes is certainly important for the growth of waves (e.g., EMIC), that can control hot particle populations.

Future Investigations

At the workshop it was decided to address our outstanding questions by selecting several specific events with good data coverage, and using these workshop-selected events (WSE) to conduct a handful of science investigations. The events chosen:

  1. the erosion of 18 June 2001;
  2. the MAXIS (19–25 January 2000) and MINIS (21–25 January 2005) balloon campaigns;
  3. selected events from the CRRES data set.

All researchers are encouraged to participate in the analysis of these events. For information, contact Jerry Goldstein (jgoldstein@swri.edu) or Joe Borovsky (jborovsky@lanl.gov).

Data from the three groups of WSE would form the baseline set of measurements for the following investigations:

  1. Ground radar and TEC maps, in conjunction with images and space-based in situ data, will be used to test the plasmasphere-ionosphere relationship and examine the global morphology of the outer plasmasphere and plumes.
  2. Various interrelated datasets (GPS TEC, IMAGE EUV, CRRES and LANL) will be employed in a statistical investigation to determine if longitudinal or seasonal effects can be identified in the plasmasphere, ionosphere or magnetosphere.
  3. Spacecraft measurements in the magnetosphere will be statistically coordinated with ground- and balloon-based (MAXIS and MINIS) precipitation measurements to compare the magnetospheric hot particle populations with the rate of precipitation to estimate outer electron belt loss coefficients.
  4. An occurrence and amplitude survey of EMIC waves will be compared with the occurrence and density of plumes to quantify the role of plumes in governing hot particle dynamics.
  5. A global computational convection model will be used to examine the fate of plumes at the dayside reconnection site.

The 2006 Plumes Workshop was held 9–13 October 2006 in Taos, New Mexico. A second workshop on plasmaspheric drainage plumes is planned for late 2007. For more information, contact jborovsky@lanl.gov.

References