1. A.B. Galvin*1, K.D.C. Simunac1, M.A. Popecki1, and C.J. Farrugia1
Solar Wind Ion Observations: Comparison from the Depths of Solar Minimum to the Rising of the Cycle
A.B. Galvin*1, K.D.C. Simunac1, M.A. Popecki1, and C.J. Farrugia1
1.Space Science Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire
*Contact:
Abstract
This latest solar minimum has been termed "peculiar" compared to earlier minima for which solar wind is available, and the prediction for this coming solar maximum is that it may be the smallest sun spot cycle in decades. We discuss observations by STEREO PLASTIC of solar wind ions’ kinetic and compositional parameters as observed during the recent solar minimum and the current rise in solar cycle, with emphasis on the differences observed in the interplanetary coronal mass ejection ion charge states.
Figure 8
ICME Iron Charge States - Cycle Trends
Of particular interest to the mission objectives of the STEREO mission are the identification of the passage of interplanetary coronal mass ejections (ICMEs). For ICME identification, PLASTIC provides the proton and alpha parameters and selected minor species.
Although high ionic charge states are a reliable indicator of an ICME (e.g., Reinard, 2008), not all ICMEs exhibit extreme composition. The ICMEs observed during this past solar minimum were typically characterized by very modest Fe charge states, as reported by Galvin et al. (2009) and Galvin (2011). Jian et al. (2011) report that the ICMEs in the recent solar minimum were smaller, slower and weaker than those observed during the 1996 minimum.
Shown in Fig. 5, are the average iron ionic charge states Fe<Q>, derived from hourly accumulations, observed by STA PLASTIC from commissioning into early As expected, the occurrence frequency of higher Fe<Q> events is increasing with the approach to solar maximum. Illustrated in the 2011 series, ICME candidates that have been identified by Jian (Level 3 ICME list, 2012) are indicated by their entry number. Those entries below the trace indicate either modest or no significant Fe<Q> signature, while those above indicate Fe<Q> greater than 12 observed. A few periods with higher charge states are not on the candidate list, and warrant further investigation. The highest Fe<Q> observed to date on STA is during the June magnetic clouds (Fe<Q> above 16). The May 28, 2012 event is also notable (Fe<Q> above 15). The past 17 months of Fe<Q> for STB is shown in Fig 6.
Figure 7
Introduction
The Plasma and Suprathermal Ion Composition (PLASTIC) instruments on STEREO A (STA) and STEREO B (STB) were commissioned in January 2007 and have been operating continuously. The mission to date encompasses the decline into the solar minimum of December 2008 and the rising portion of cycle 24 (Fig. 1). The prediction for the next solar maximum is for a small peak in the sun spot number. The PLASTIC investigation measures the solar wind protons, alphas, and selected minor ions.
Compositional changes in the solar wind during the solar cycle has been well established for helium (Ogilvie et al., 1974), with a dependence on speed (Aellig et al., 2001) and latitude (Kasper et al., 2007) having been established, particularly for slow (non-coronal hole) solar wind.
Given the unusual depth of the recent solar minimum, and the anticipated nature of the size and arrival date of the coming solar maximum, this cycle is of interest in studying how the different cycle structure affects the composition within the solar wind. For helium, this recent solar minimum exhibited a 50% lower He/H value than last solar minimum, using the OMNI data set (Fig. 2, Table 1).
Table 1. Comparison of two solar minima
<SSN> <He/H>
July
June 2009
As shown in Fig. 3, the He/H solar wind abundances observed by STEREO A indicate that the cycle variation is apparent for all data without selection, density selection (1< Np < 10 cm-3); density selection with elimination of identified ICME periods. All show the same general trend of higher He/H vs. sunspot number. (Note: we have not at this time separated data by speed or latitude, suggested as appropriate by the Aellig et al. and Kasper et al. results).
Figure 1
STEREO
STA June ICME and Magnetic Cloud Event
Superimposed on the PLASTIC overview (Figure 7) for this event are the preliminary start and stop times determined by Jian (2012) for the ICME candidate event (grey bar) and the two associated magnetic structure passages (red bars) for the June 6-7 time period. Proton speeds measured by PLASTIC reached nearly 1000 km/s. As seen in Figure 8, the times with higher Fe charge states encompass the nominal ICME interval, but extend several hours beyond.
Figure 5
Figure 2
Above: Iron charge state distributions (hourly) for June 6, 7, 8 which clearly show the arrival and end times for the “hot” plasma.
Figure 9
STB Oct ICME and Magnetic Cloud Event
Superimposed on the PLASTIC overview (Figure 9) for this event are the preliminary start and stop times determined by Jian (2012) for the ICME candidate event (grey bar) and the associated magnetic structure passages (red bar) for the Oct time interval. Proton speeds measured by PLASTIC reached about 700 km/s. As seen in Figure 9, the times with higher Fe charge states nicely encompass the nominal ICME interval.
The STEREO B iron charge state distributions in Fig 10 (rotated here from the perspective of Fig 8) clearly show the time line of the low charge states being dominant prior and subsequent to the passage of the ICME.The high charge states dominate during the ICME interval, and are absent elsewhere.
In Figure 11, matrix elements over significant mass groupings are shown for a 10-hour period on Oct 4th. The Nq values are logarithmic values of mass/charge.
Figure 3
Figure 4. The He/H solar wind abundances observed by STEREO A from , all data (no selection criteria), plotted against sun spot number.
Figure 10
Figure 6
References
Aellig, M.R., A.J. Lazarus, and J.T. Steinberg, The solar wind helium abundance: variation with solar wind speed and the solar cycle, Geophys. Res. Letters, 28, , 2001.
Galvin, A.B., et al., Solar Wind Trends and Signatures: STEREO PLASTIC Observations Approaching Solar Minimum, Ann. Geophys., 27, , 2009.
Galvin, A.B., Solar Wind Observations from the STEREO Perspective ( ), Chapter 11 in The Sun, the Solar Wind, and the Heliosphere, IAGA Special Sopron Book Series 4, ed. M.P. Miralles and J. Sanchez Almeida, Springer, pp , doi / _11, 2011.
Galvin et al., STEREO-4 and SOHO-25 Workshop, Kiel, Germany, July 2011.
Jian, L.K., C.T. Russell, and J.G. Luhmann, Comparing solar minimum 23/24 with historical solar wind records at 1 AU, Solar Phys., 274: , 2011.
Jian, L.K.,
Kasper, J.C., et al, Solar wind helium abundance as a function of speed and heliographic latitude: variation through a solar cycle, Astrophysical Journal, 660: , 2007.
Ogilvie, K.W. and J. Hirshberg, The solar cycle variation of the solar wind helium abundance, Journal of Geophys. Res., 79, 4595, 1974.
Reinard, A.A., Analysis of interplanetary coronal mass ejection parameters as a function of energetics, source location, and magnetic structure, Astrophysical Journal, 682: , 2008.
Fig. Charge groups are distinguished in the PLASTIC data through separation on mass. Due to the low operational voltage and other factors, the less dominant species require fitting routines. The red trace is all elements that had no mass determination (energy falls below detector threshold), and includes protons and much of the helium.
Figure 11
Acknowledgments
NASA STEREO Contract NAS at UNH
IMPACT data and analysis courtesy J. Luhmann, C. Russell, and most particularly for her level 3 lists: L. Jian.
Omni data from the NASA data center