Solar Wind Cross Helicity in Solar Cycles 23 and 24
Explore the relationship between solar wind cross helicity and Alfvénicity over solar cycles 23 and 24. Learn how cross helicity can approximate Alfvén wave content, particularly in high-speed solar wind streams, for potential space weather applications. Estimation methods and data from the ACE spacecraft at L1 are discussed, along with lists of Alfvénic events.
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Solar wind cross helicity over solar cycles 23 and 24 2018.03.07 Reko Hyn nen1, E. I. Tanskanen1, 1Aalto-yliopisto Reko.Hynonen@aalto.fi
Motivation: Cross helicity vs. Alfv nicity and geoefficiency Cross helicity may be used to approximate the Alfv n wave content of solar wind. Alfv nicity is a property usually attributed with high- speed solar wind streams. For space weather purposes in the high latitudes, high speed streams often seem to be more efficient than other solar wind structures. 10.5.2025 2
Motivation: Cross helicity vs. Alfv nicity and geoefficiency 10.5.2025 3
Estimating Alfvnicity Following Snekvik et al. (2013) and Tu & Marsch (1994): Els sser variables ?? ?? = ?? = ?? ??? 4?? Total spectra ?? =2 ? (???,? ) ?? ???,? ? Total energy spectra (+) and cross helicity spectra (-) ? ?? =1 ???? =1 Normalized cross helicity ???? =???? ?+?? ? ?? 2 ?+?? ? ?? 2 require ??> 0.8 of IMF-parallel waves for Alfv nicity ? ?? 10.5.2025 4
Data: ACE spacecraft Stationed at L1 Importantly, measures: IMF at 1-sec resolution SW speed and density at 64-sec resolution We pick samples of 256 points at 64-sec resolution (~4.55 hrs) 10.5.2025 5
List of Alfvnic events (ALFs) 10.5.2025 6
List of Alfvnic events (ALFs) 10.5.2025 7
An example long event: May 2004 10.5.2025 8
Another example: February 1999 10.5.2025 9
Some properties of the event list 10.5.2025 10
Some properties of the event list 10.5.2025 11
Some properties of the event list 10.5.2025 12
Some properties of the event list 10.5.2025 13
Mean SW speed during ALFs 10.5.2025 14
Mean IL during ALFs Naive comparison but promising 10.5.2025 15
Conclusions Pros of the method Cross helicity is relatively easy to compute. Need only 1 spacecraft in solar wind Wind, Messenger, DSCOVR Potential for future/workshop Event analysis Substorms, geomagnetic storms, comparison to known highspeed streams We need some theoretical help/pointers! E.g. is there something specific use for the data or something to watch for? 10.5.2025 16
