Interplanetary Field During the Current Solar Minimum

Transcription

1 Interplanetary Field During the Current Solar Minimum C.T. Russell 1, L.K. Jian 1, J. G. Luhmann 2, T.L. Zhang 3 1 UCLA, 2 UCB, 3 SRI, OEAW SOHO 23 Understanding a Peculiar Solar Minimum Asticou Inn, Northeast Harbor, Maine Friday, September

2 The Changing Interplanetary Magnetic Field The IMF generally weakens slightly during solar minimum as illustrated here for the last 3 solar cycles. This was true of the current solar minimum but when the extended minimum proceeded the field strength continued to fall. The field strength weakened in the equatorial regions and in the high latitudes simultaneously (Smith and Balogh, 2008) 2

3 Proximate Cause of Weak IMF: Weak Photospheric Field Top panel shows the signed emerging flux in the photosphere for last 3 cycles Bottom panel shows the unsigned emerging flux seen by WSO In the last cycle not much flux emerged into the photosphere 3

4 Result of Weak Photospheric Field: Low Transport To Poles Dave Hathaway has illustrated how the polar transport of flux weakened in cycle 23. Naively lower transport would result in a longer time for the field reversal and that certainly has happened but more than just a longer polar reversal time ensued. 4

5 Result of Low Rate of Flux Transport to Poles: Weak Polar Fields This shows the WSO observed polar field strength. It has dropped more than a factor of two. Is this the cause of the weak fields in the IMF? No, because the field comes from low latitude coronal holes as well. The solar field is weak everywhere. 5

6 Neutral Sheet Location: Evolving Dance of the Ballerina The location of the neutral line changes greatly during the solar cycle Current sheet is highly inclined at solar maximum and only slightly inclined at solar minimum Over the last 3 solar cycles pattern was quite similar until the slow decline to solar minimum Does this affect stream interactions? 6

7 Coronal Holes at Low Latitudes: PFSS Maps Near Minimum PFSS maps show where field lines are open and where they are closed These maps are selected from the declining phase of solar cycle 23 They reveal low latitude coronal holes that are a source of the solar wind and IMF As a result of these low latitude sources, stream interactions and weak shocks have continued through solar minimum 7

8 Solar Minimum Phenomena: Interplanetary Shocks While the current sheet has become flatter, this has not stopped fast streams overtaking slow streams. Here are two shocks a forward and a reverse shock produced by fast streams overtaking slow streams. These shocks, as are all shocks recently, are weak. ICMEs generally do not have a leading shock at this solar minimum 8

9 Solar Minimum Phenomena: Mirror Mode Storms Many phenomena are present in the solar wind at this solar minimum and there is lots of plasma physics to explore. Mirror mode storms occur in high beta regions. These waves consist of very squarish peaks and dips. These may evolve into deep isolated mirror mode structures. 9

10 Solar Minima Phenomena: Ion Cyclotron Waves Ion cyclotron waves have been discovered in the solar wind. They are produced in the corona and significantly heating the solar wind (L. K. Jian, ApJL., 701, 2, L105 L109, 2009) These waves extend from inside 0.3 AU to at least 1.0 AU (Lan Jian, poster #25) If the IMF continues to weaken they will all be absorbed inside 1 AU 10

11 Solar Minimum Phenomena: Interplanetary Field Enhancements Interplanetary field enhancements are sharp field enhancements with a strong central current sheet. They are seen throughout the inner solar system from 0.3 AU to beyond 1 AU They travel at the speed of the solar wind and may be associated with the acceleration of dust. They seem to be unaffected by the deep solar minimum. 11

12 Solar Minimum Phenomena: Periods of Very Weak Fields The magnetic field can become very weak for periods approaching a day For short periods the field can be very close to zero. LH panel shows 24 hr and RH panel shows 4 minutes at 0915 nt. Since the magnetic field couples energy to the Earth s magnetosphere these conditions result in very weak geomagnetic activity. 12

13 Magnetic Field at Solar Minimum at 0.72 AU Magnetic field strength at 0.72 AU as seen by Venus Express has been dropping just as the field at 1 AU and at Ulysses has been dropping. Rate of decrease since January 2006 has been 0.6 nt per year. 13

14 Are Such Low Fields Unprecedented? One hour and three hour indices have been used to derive the interplanetary magnetic field strength for over a century by Lockwood et al. (2009) Where they co exist the observations and predictions are very similar so we may trust these predictions They tell us that in the last 100 years we have not seen such low field strengths 14

15 Summary and Conclusions The strength of the interplanetary magnetic field has been decreasing since This change can be seen at the orbits of Venus and Earth and out of the ecliptic plane. The proximate cause of the decline is the weak photospheric magnetic field. The heliospheric current sheet has become equatorial but low latitude coronal holes have maintained stream interactions While strong ICMEs have disappeared other solar wind phenomena such as shocks, mirror mode waves, ion cyclotron waves, and interplanetary field enhancements have not. Using geomagnetic activity as a proxy for the strength of the IMF we infer that such low IMF strengths have not occurred during the past 100 years. 15