Ann. Geophys., 33, 845–855, 2015 www.ann-geophys.net/33/845/2015/ doi:10.5194/angeo-33-845-2015 © Author(s) 2015. CC Attribution 3.0 License. Solar-wind control of plasma sheet dynamics M. Myllys1, E. Kilpua1, and T. Pulkkinen2 1Department of Physics, University of Helsinki, Helsinki, P.O. Box 64, Finland 2Aalto University School of Electrical Engineering, P.O. Box 1100, 00076 Aalto, Finland Correspondence to: M. Myllys (minna.myllys@helsinki.fi) Received: 24 February 2015 – Revised: 21 May 2015 – Accepted: 22 May 2015 – Published: 21 July 2015 Abstract. The purpose of this study is to quantify how solar- 1 Introduction wind conditions affect the energy and plasma transport in the geomagnetic tail and its large-scale configuration. To iden- tify the role of various effects, the magnetospheric data were The coupling of the solar wind to the magnetosphere has long sorted according to different solar-wind plasma and inter- been a subject of active research. In particular, the mech- planetary magnetic field (IMF) parameters: speed, dynamic anisms of the energy transfer from the solar wind to the pressure, IMF north–south component, epsilon parameter, magnetosphere have been of great interest. According to the Auroral Electrojet (AE) index and IMF ultra low-frequency open model of the magnetosphere (Dungey, 1961), the mag- (ULF) fluctuation power. We study variations in the average netospheric magnetic field reconnects with the interplane- flow speed pattern and the occurrence rate of fast flow bursts tary magnetic field (IMF) on the dayside magnetopause. The in the magnetotail during different solar-wind conditions us- opened geomagnetic field lines are transported to the magne- ing magnetospheric data from five Time History of Events totail, where they are reconnected again. The resulting closed and Macroscale Interactions during Substorms (THEMIS) and stretched field lines migrate back to the dayside magne- mission spacecraft and solar-wind data from NASA’s OM- topause. This motion generates the large-scale plasma con- NIWeb. The time interval covers the years from 2008 to vection in the magnetosphere. 2011 during the deep solar minimum between cycles 23 and The energy transfer efficiency from the solar wind to the 24 and the relatively quiet rising phase of cycle 24. Hence, Earth’s magnetosphere is often studied by comparing vari- we investigate magnetospheric processes and solar-wind– ous coupling functions (combinations of solar-wind param- magnetospheric coupling during a relatively quiet state of the eters), as well as individual solar-wind parameters, with the magnetosphere. We show that the occurrence rate of the fast geomagnetic indices (Meng et al., 1973; Stamper et al., 1999; −1 Finch and Lockwood, 2007). For example, Akasofu’s epsilon (jVtailj> 100 km s ) sunward flows varies under different solar-wind conditions more than the occurrence of the fast parameter (Perreault and Akasofu, 1978; Akasofu, 1981) is a tailward flows. The occurrence frequency of the fast tailward commonly used coupling function to estimate the efficiency flows does not change much with the solar-wind conditions. of the energy transfer to the inner magnetosphere. The day- side reconnection rate depends strongly on the sign of the We also note that the sign of the IMF BZ has the most visible effect on the occurrence rate and pattern of the fast sunward IMF north–south component (BZ component). In addition, flows. High-speed flow bursts are more common during the the solar-wind speed has been found to correlate well with slow than fast solar-wind conditions. monthly and annual geomagnetic activity levels (Crooker et al., 1977; Finch et al., 2008). Keywords. Magnetospheric physics (plasma sheet) Plasma sheet flows play a central role in the magneto- spheric energy transport. The earliest studies focused mostly on the plasma sheet flow properties and studied the occur- rence rate and spatial distributions of the fast and slow ion flows (Angelopoulos et al., 1992, 1993; Baumjohann et al., 1990; Shiokawa et al., 1997; Wang et al., 2006). Most of these studies defined the threshold of the high-speed flow Published by Copernicus Publications on behalf of the European Geosciences Union. 846 M. Myllys et al.: Solar-wind control of plasma sheet dynamics −1 to be 400 km s . The bursty bulk flow events (BBF) (An- −15>X > − 30 RE (Ohtani et al., 2009). The dominance of gelopoulos et al., 1992, 1993) are defined as periods when tailward flows with northward BZ indicates that reconnection the flow speed average exceeds 100 km s−1 and is above is not the primary cause of the flows. Ohtani et al.(2009) sug- 400 km s−1 (Baumjohann et al., 1990; Angelopoulos et al., gest that the sources of tailward flows could be the balloon 1992) for at least two samples during the 1 min period. instability and rebound of fast sunward flows near the Earth. High-speed (> 250 km s−1) flows have also been classified The latter mechanism is supported by the fact that tailward into two distinct categories based on their ion distributions: flows are normally preceded by fast sunward flows and also bulk flows and field-aligned beams (Nakamura et al., 1991; by global Magnetohydrodynamics (MHD) simulations (Wilt- Raj et al., 2002). Bulk flows are perpendicular to the mag- berger et al., 2000). netic field at the neutral sheet but have a large field-aligned The purpose of this study is to quantify how solar-wind component at higher magnetic latitudes. Field-aligned beams conditions affect the energy and plasma transport in the are mainly detected away from the neutral sheet. The occur- geomagnetic tail. We use tail observations from the five rence of BBF shows a dawn–dusk asymmetry in contrast to THEMIS spacecraft during the years 2008–2011. During this field-aligned beams (Raj et al., 2002). period the THEMIS spacecraft spend a considerable time in In recent studies, the average bulk flow pattern and the the geomagnetic tail allowing us to compile statistical maps characteristics of the plasma sheet flows with speed have of plasma flow properties and to study the tail energy trans- been studied during the different IMF BZ conditions and sub- port. In addition, this time period corresponds to the ex- storm phases (Juusola et al., 2011a,b). However, there are tended and prolonged solar activity minimum between solar only few studies of the large-scale magnetospheric flow pat- cycle 23 and 24 and the relatively quiet rising phase of cycle tern under different solar-wind conditions. One such study 24. This allows us to investigate magnetospheric processes was recently published by Pulkkinen et al.(2013), who stud- and solar-wind–magnetospheric coupling during a relatively ied plasma sheet flows during steady magnetospheric con- quiet state of the magnetosphere. To uniquely identify the vection events. role of various effects, the magnetospheric data were sorted Previous studies of plasma sheet convection have shown according to different solar-wind plasma parameters and by that (1) the average ion bulk flow speed in the central plasma the Auroral Electrojet (AE) index. We study the variations in sheet has been found to be low, below 100 km s−1 (Baumjo- the average bulk flow speed pattern and the occurrence of the hann et al., 1990; Juusola et al., 2011b); (2) high-speed flows flow bursts exceeding 50 and 100 km s−1 and their preferred (> 400 km s−1) occur in bursts mostly less than 10 s in du- direction during different solar-wind conditions. ration (Baumjohann et al., 1990); (3) the slow flow pat- In this paper the first section describes the data sources and −1 tern (jv?j<200 km s ) is not significantly different dur- methods used in this study. The second section analyses the ing northward and southward IMF conditions, but the over- results. We end with discussion and conclusions. all flow speed is higher during southward IMF (Wang et al., 2006); (4) the largest occurrence rates of high-speed bursts (> 400 km s−1) are found near the midnight merid- 2 Data and methods ian, and their occurrence peaks strongly in the sunward di- 2.1 Data sources rection (Baumjohann et al., 1990); (5) high-speed bursts −1 (> 400 km s ) are almost always directed earthward in- The solar wind and AE index data with 1 min resolution − D side 20 RE (Earth radius 6372 km), indicating that their have been extracted from NASA Goddard Space Flight Cen- − source is beyond X (GSM)< 19 RE (Shiokawa et al., ter’s OMNI data set through the OMNIWeb interface (http: 1997); (6) the occurrence rate of the high-speed burst de- //omniweb.gsfc.nasa.gov/). The data have been propagated D − − creases closer the Earth in the region from X 19 to 9 RE to the nose of the Earth’s bow shock. The parameters used (Shiokawa et al., 1997); (7) the tailward BBFs are rare and in- are the solar-wind flow speed, IMF components in the GSM frequent but the ratio of tailward to earthward BBFs increases (geocentric solar magnetospheric coordinate system) coordi- with distance from the Earth (Angelopoulus et al., 1994); (8) nates, dynamic pressure and AE index. BBF events occur in only 10 % of crossings (Angelopoulus The magnetospheric data come from the five THEMIS et al., 1994). spacecraft. The THEMIS mission consists of five identically Although on average the plasma flows are directed sun- instrumented satellites orbiting the Earth at different dis- ward in the plasma sheet, there are also tailward flows. The tances. The orbits cover the magnetotail from 4 to 30 RE. The tailward flows can be classified depending on their BZ ori- ion velocity and density measurements come from the ESA entation. Tailward flows with southward BZ are rare and instrument (McFadden et al., 2008) and the spin-averaged are usually considered to be caused by reconnection tak- magnetic field measurements come from the Flux Gate Mag- ing place earthward of the observation point.