HOP 0139

abstract of observational proposal

Prominences (also called Filaments) are among the most enigmatic structures in the solar atmosphere. Prominences form in both active regions and quiet Sun regions and are accordingly referred to as active-region prominences (ARPs) and quiescent prominences (QPs). The formation mechanism(s), dynamic evolution, and causes of eruption of prominences all remain active areas of research in solar physics. While it is known that prominences form over Polarity Inversion Lines
(PILs) in the photospheric magnetic field configuration, it is not known how the magnetic field configuration which determines the morphology of the prominence originates, nor is it known how the prominence mass comes to populate the field lines. Measurements show that the horizontal magnetic field is highly sheared to lie approximately 15° from the PIL, but a central debate in prominence physics is whether the shear is produced by flowfields acting on potential-like fields crossing the PIL or whether it arises “pre-sheared” in the form of helical magnetic flux ropes.

From an excellent time series of photospheric vector magnetic fields taken with the SOT/SP, Okamoto et al. (2008, 2009) found clear observational evidence of an emerging helical flux rope below an ARP. The length of the emerging flux rope was inferred to be 30,000 km or more. Lites et al. (2009) also obtained similar results showing the emergence of the flux rope in AR filament channel within another active region. While this is strong evidence supporting the flux rope hypothesis, it remains to be seen whether these emergence events consistently occur before the formation of a filament – Hinode has yet to observe the beginning phases of filament formation in spite of three years of operations.

Another central question in prominence research is what causes the obvious differences in ARPs and QPs. Hinode/SOT observations have verified that while all ARPS are horizontally structured, both when seen on the disk and when seen at the limb, QPs appear horizontally structured on the disk but always appear vertically structured when seen at the limb. In addition, Hinode/SOT observations have discovered extremely strange vertical buoyant flows in QPs but
these flows are not seen in APRs (Berger et al. 2008, 2009). Is the stronger magnetic field strength in ARPs solely responsible for these dramatic differences or is there another factor at work, perhaps involving the emerging flux rates below these structures? Finally, both ARPs and QPs are known to erupt in CMEs. These eruptions appear to have a helical structure in low-resolution observations thus leading to the conjecture that they remove significant amounts of helicity from the global magnetic field, yet Hinode has yet to capture a filament eruption to measure any noticeable before and after changes in the photospheric magnetic field.

This HOP aims to understand the formation, dynamic evolution, and eruption of prominences, both in active regions and in quiet Sun regions. A major issue for prominence studies is the small number of data sets available to investigate the evolution of photospheric magnetic fields below prominences. Currently we have only two data sets for AR prominences, and no SP vector field datasets for QPs. More observations of prominences with Hinode/SOT are necessary for better
understanding the prominence formation/evolution.

We thus propose two types of prominence observations:
(1) Large FOV, long-duration, vector magnetic velocity measurements of filaments on the disk. We will track either ARPs or QPs (depending on target availability) across the disk for several days in order to examine the evolutional history of photospheric magnetic and velocity fields below the prominence during its lifetime. Many data sets of this type will allow
us to confirm whether the prominence formation due to the emerging flux rope is common to AR prominences or not. Moreover, we will look for on-disk signatures of the strange buoyant flows seen in QPs and try to relate them to magnetic or velocity perturbations in the photosphere. The evolution of photospheric magnetic fields during and after the eruption of the prominence is also essential to measure in order to understand the lower atmospheric changes brought on (if any) by CMEs.
(2) Small FOV, short duration, high cadence observations of filaments on the disk. The evolution of horizontal magnetic fields near the PIL indicate the development of the so-called “filament channel” that is required for prominence formation. Such observations will give us detailed evolution of the horizontal magnetic fields and convective flows around the PIL at a cadence high enough to see the detailed evolution of the filament channel magnetic fields.

In our proposed observations, the SOT/SP is most important for investigating the evolution of photospheric magnetic field vector related to the formation of prominences. If a flux rope emerges into the corona through the photosphere, we can infer the 3D structure of prominence magnetic fields from a time series of the cross sections between the emerging flux rope and the photosphere (like a physical examination by CT scan). According to the previous study in Okamoto et al. (2008), the upper half of the flux rope took about 6 hours to pass through the photosphere, and the bottom half took about 12 hours. In this observation, the SP maps were taken every 1―6 hours for 2 days. We want to improve both the cadence and duration of these observations: a cadence less than 2 hours and duration of more than several days is required to make progress on the key prominence questions outlined above. In addition to SP observations, H-alpha observations are needed in order to show the on-disk location and evolution of prominences. We request to take H-alpha images and Na I D magnetograms simultaneously when we can identify a clear, large prominence in the non-eclipse season. In the other cases, we request only Na I D magnetograms. Ca II H images are also needed for distinguishing strong vertical fields in the photosphere with weaker fields.

request to SOT

<Pointing>
Please set the pointing at the center of a prominence in the H-alpha image or at the center of the polarity inversion line (not sunspot center) in the magnetogram.

(1) Case 1 (standard)

SP - Fast map (cycle 2), FOV=100”x110”, sum=2x2, cadence=1.5 hours
→ 2100 Mbits/day
(FOV depends on the location and length of polarity inversion line)

H-alpha center - FOV=160”x160”, sum=2x2, cadence=10 minute, Q=75
(only a clear prominence in the non-eclipse season)
→ 310 Mbits/day

Na IVDG - FOV=160”x160”, sum=2x2, cadence=10 minutes, Q1=65, Q2=75
(SP has the highest priority, so reduce the cadence and FOV if telemetry does not allow)
→ 900 Mbits/day

G-band - FOV=160”x110”, sum=2x2, cadence=1.5 hours, Q=65
(for alignment purpose)
→ 75 Mbits/day

Ca - FOV=110”x110”, sum=2x2, cadence=10 minute, Q=65
(if telemetry allows)
→ 310 Mbits/day

Total = 3700 Mbits (current SOT allocation : 3500 Mbits/day)

(2) Case 2 (high cadence)

SP - Fast map (cycle 2), FOV=20”x82”, sum=2x2, cadence=repeat,
→ 285 Mbits/hour

H-alpha center - FOV=80”x80”, sum=2x2, cadence=4 minute, Q=75
(only a clear prominence in the non-eclipse season)
→ 8 Mbits/hour

Ca - FOV=80”x80”, sum=2x2, cadence=4 minute, Q=65
→ 18 Mbits/hour

G-band - FOV=80”x80”, sum=2x2, cadence=1 minute, Q=65
(A 1-min cadence is necessary for the calculation of the surface flows.)
→ 75 Mbits/hour

Na IVDG - FOV=80”x80”, sum=2x2, cadence=4 minutes, Q1=65, Q2=75
→ 24 Mbits/hour

Total = 400 Mbits/hour (A duration of 3-4 hours or more is expected)

remarks

We request to perform our HOP repeatedly using the following rules in order to increase the number of prominence observations:
(1) When COs notice a prominence on the disk (within about 800” from the disk center), please start this HOP as soon as possible.
(2) In the case of a prominence located at the East limb, please run this HOP one day after the prominence has transited onto the disk.
(3) Coordination with HOP73 (Quiescent Prominence Dynamics by Berger et al.) and HOP114 (Prominence Cavities by Schmidt et al.) is important. When you run these HOPs for prominences/cavities at the East limb, please follow with this HOP. Conversely, when you run this HOP on a filament toward the West limb, please follow with HOP 73 (or 114 if the coronal cavity is very clear in EUV images) when the prominence appears over the limb.

If you do not receive any requests from T. J. Okamoto, please perform “Case 1 (standard)”observation. Once this observation starts, we require “continuous” observation for more than 4 days (except XRT synoptic observation or short break less than 30 minutes) even if the targeted prominence disappears. We note that we can stop the observation and postpone it if a large sunspot appears during this observation.