Eija Tanskanen

Magnetospheric energy budget and the epsilon parameter

[1]xa0Determination of the energy input for the magnetospheric energy budget is a nontrivial matter. As no direct means to measure the input are known, various solar wind-derived proxies have been developed. In this article we discuss one of the most widely used energy input functions, the so-called epsilon parameter of Akasofu. While practice has shown it to be a very useful parameter, there is no convincing evidence that it is superior to all other coupling parameters. Furthermore, its somewhat unclear definition and lack of physical foundation sometimes lead to confusing interpretation of the parameter in practical studies of magnetospheric energy cycle. For example, the parameter is sometimes understood to describe the transfer of solar wind Poynting flux into the magnetosphere, whereas the actual physical energy transfer involves conversion of solar wind kinetic energy to magnetic energy measured inside the magnetopause. Another questionable interpretation is to relate the size of the energy transfer region to the length of the reconnection line, as the scale factor in epsilon has the physical unit of area. These confusions may partly result from mixing the concepts of energy source and energy transfer. In spite of these problems the present empirical formulation of the epsilon parameter appears, from the global energy budget point of view, to give a remarkably good estimate for the total energy input into the inner magnetosphere in substorm and storm timescales. This is even more remarkable as after the parameter was first formulated we have learned that the ionosphere is a major sink of storm and substorm energy, exceeding the ring current in importance as an energy output channel. An additional issue is the energy carried away by the plasmoids and outflow of the postplasmoid plasma sheet. One can argue that the application of epsilon should be restricted to the energy consumption in the inner magnetosphere. However, as the intermittent plasmoid releases are essential parts of the same complex of processes as the ring current enhancement and ionospheric particle injections, we argue that they should be included in the energy budget, even if that might result in rejection of the epsilon as a useful input parameter. The recent analyses of energy output suggest that we can still use epsilon by scaling the parameter up by a factor of 1.5–2. It should be noted, however, that this energy budget does not account for all energy passing through the magnetosphere but only that part which is consumed in the storm and substorm processes.

Cluster observations of traveling compression regions in the near-tail

[1]xa0Examination of Cluster measurements has revealed the presence of traveling compression regions (TCRs) in the lobes of the Earths magnetotail at X ∼ −11–19 RE. These TCRs strongly resemble those observed in the more distant tail, but their mean duration is only ∼35 s as compared with ∼160 s for the TCRs in the distant tail. Furthermore, the Bz variations associated with the Cluster TCRs were found to be south-then-north (SN) in 80% of the cases as opposed to the north-then-south (NS) polarity that is dominant beyond X ∼ −30 RE. Analysis of the time of arrival of the TCRs at the different Cluster spacecraft showed that all of the SN TCRs propagate earthward while all of the NS TCRs, as expected, move tailward. The mean speeds of the SN and NS TCRs were essentially the same, 849 km/s and 821 km/s, respectively, and their average width was 4.3 RE. Some examples of near-periodic, multiple TCR events with separations between individual TCRs comparable to their width were also observed, suggestive of multiple X-line reconnection or periodic impulsive reconnection. However, the most probable separation observed during multi-TCR events was larger, ∼100–150 s. The TCR minimum variance eigenvectors have a strong tendency to lie parallel to the GSM XY plane, but they exhibit a wide range of orientations within that plane. Examined as a function of the YGSM, there are broad maxima in occurrence frequency, width, and speed of TCRs on the duskside of the tail. Superposed epoch analysis of the Kyoto World Data Center Quick Look AL Index relative to the time of TCR occurrence shows that the compression regions tend to be observed during the expansion phase of substorms. Finally, the origins of the traveling compression regions in the near-tail are discussed in terms of the effects of magnetic flux rope motion and impulsive reconnection.

Magnetospheric substorms are strongly modulated by interplanetary high-speed streams

[1]xa0The occurrence of substorms was examined over a complete 11-year solar cycle, identifying over 5000 substorms. It was found that high-speed streams strongly modulate the substorm occurrence rate, peak amplitude and ionospheric dissipation in the form of Joule heating and auroral electron precipitation. Substorms occurring during the years of frequent interplanetary high-speed streams (1994 and 2003) are 32% more intense, on average, and transfer twice as much magnetic energy to the auroral ionosphere as the substorms occurring during the years of few or no high-speed streams (1993, 1995–2002). To characterize and to predict the substorm activity we form a new measure, the substorm activity parameter Rsu, which we expect to become a powerful tool in analyzing the near-Earth space climate.

Magnetotail response to prolonged southward IMF Bz intervals: Loading, unloading, and continuous magnetospheric dissipation

[1]xa0The response of the Earths magnetotail to prolonged southward interplanetary magnetic field (IMF) has been determined for the three Geotail magnetotail seasons from November to April, 1999–2002. We examine the total magnetotail pressure PT,tail = B2/2μ0 + NikTi because variations should be similar in this parameter in the lobes and in the plasma sheet. We found 13 events when IMF Bz remained southward for 8 hours or longer and Geotail was located within the magnetotail farther than 10 RE downstream. All 13 events were subdivided into separate intervals characterized as (1) loading, if the tail total pressure increased more than 100%; (2) unloading, if the total pressure decreased by more than 50%; and (3) what we term here continuous magnetospheric dissipation (CMD), if the tail total pressure increased by less than 100% and/or decreased less than 50% during the entire mode interval. In total, 37 loading, 37 unloading, and 28 CMD events were found. The plasma sheet magnetic flux transfer rate, ϕEarth ≈ vx · Bz, and plasma bulk velocity has been analyzed to determine the steadiness of the plasma sheet convection. Plasma sheet convection was found to be highly disturbed and intense plasma flows (BBFs and FBs) were observed during all convection states. However, the occurence rate and amplitude of plasma flows distinguish loading-unloading and continuous dissipation periods from each other. BBFs seem to be more numerous (135) but weaker (about 500 km/s) during continuous dissipation intervals compared with BBFs existing during unloading mode (61 and 660 km/s). Finally, it was found that CMD-type convection is more likely when the mean southward IMF Bz > −5 nT, while loading-unloading is more likely when IMF Bz < −5 nT.

Cluster four spacecraft measurements of small traveling compression regions in the near-tail

Cluster four spacecraft measurements of small travelling compression regions in the near-tail

Auroral electrojet configuration during substorm growth phase

[1]xa0The spatial configuration of the auroral electrojets during the growth phase of classical substorms is investigated. Electrojet intensities are determined from measurements of the convection electric field and calculated height integrated conductivity. Both the westward and eastward electrojets decrease in latitudinal width towards ∼21–23 MLT where they both are terminated. The latitudinal widths of the eastward electrojet as a function of MLT, however, show significant scatter. This scatter is shown to be effectively minimized by organizing the data by the local time distance to the future optical onset location rather than using magnetic local time. We find that the future optical onset will be located in the region of overlapping eastward and westward electrojets. The fact that the future onset location better organizes the data suggests that during the growth phase the magnetosphere is organizing itself for an onset at an already determined MLT as seen at ionospheric altitudes. Neither the IMF By nor IMF clockangle provide a simple explanation of the variation in the onset location.