1. The Structure of the HPS
The in situ Density Story
Nicholeen Viall
NASA GSFC

2. Streamers Continuously Produce Periodic Density Structures
Viall & Vourlidas 2015
Viall et al. 2010
Streamers are highly dynamic, creating density variability in the form of periodic density structures
Characteristic Periodicity ~ 90 minutes; Characteristic sizes ~ 1000 Mm
Seen further out in HI1 too
In situ they occur all the time (70% to 80% of slow solar wind, Viall et al. 2008)
At May SPP SWG, showed density structures associate with helmet streamers (SECCHI images) and at the HCS (in situ Wind/Ace)

3. Flying Through a Train of Periodic Density Structures
HSC crossing filled with train of ~90 minute periodic density structures
Velocity ~constant
Alphas, C, O, charge state and B (classic flux rope!)
together indicate magnetic reconnection (S-web) in solar corona created these structures
Kepko, Viall, Antiochos, Lepri, Kasper 2016

4. We set our undergrad summer interns loose to find density structures with composition signatures that were NOT at the HCS

5. Wind/ACE see SI and periodic density structures with no HCS
Towards sector
Away sector
Interesting proton density structures Aug
Well separated from SI
Velocity constant at ~320 km/s
14 days without a HCS

6. Two-day Zoom-in on Periodic Density Structures
Multi-scale event
~12 hour proton enhancements anticorrelated with a ~12 hour alpha enhancement
~3 hour periodic density structures with alpha composition signatures

7. Use STEREO to Piece Together 3-D Picture and ID Solar Source

8. STEREO B sees ~3 hr periodic density structures at (rotationally adjusted) same time as Wind/ACE

9. STEREO A ALSO sees periodic density structures at (rotationally adjusted) same time as Wind/ACE

10. Solar source (not at the HCS) continuously released density structures for at least 6 days

11. Look to The Sun ~ 5 days Prior
Closest to viewing Earth-Sun line
Helmet streamer is depressed, consistent with no HCS crossing

12. PFSS as viewed by STEREO A
PFSS confirms HCS is depressed and aimed south of Earth
PFSS as viewed by STEREO A
PFSS as viewed by STEREO B
Closest to viewing Earth-Sun line

13. Neither helmet streamer nor Pseudostreamer plasma directed towards ecliptic

15. Antiochos et al. 2011
Periodic Density Structures
Stream Interface

16. Highly variable/structured slow solar wind density at 1 AU; characteristic plasma release ~ a few hours
Alpha composition signatures confirming solar source of variability
Solar source of periodic density variability persists for at least 6 days
Neither helmet streamer/HCS nor pseudostreamer seems to be the source…
Need full 3-D topology (e.g. reconnection at S-web corridors) to explain
Immediate Future: High res ACE composition from Sue; statistics on SECCHI periodicities from pseudostreamer and helmet streamer at that time
SPP Future: These structures ~1000 Mm too small to follow in imagers from release to S/C impact. Will see rxn.

19. 30 Minute Periodic Density Structures with Alpha/Proton Abundance Ratio Changes
a) A clear 30-minute (~850 Mm) periodic number density structure
b) Detrended time series to identify more clearly the 30-minute periodicity - they are in anti-phase
(no ACE yet)
Viall et al., 2009