This proposal requests continuous (24 hours/day) observations of a low latitude coronal hole for a single, complete OP period (either 2 or 3 days). A significant change to the standard Hinode data recorder allocations is requested (see below).
The aim of this HOP is to study the properties of magnetic features within a coronal hole. It is stimulated by recent SDO/AIA images of the large, extended north polar coronal hole that has been present for the lifetime of SDO. The following features are noted from these data:
EIt is only by observing the low latitude regions of a polar coronal hole that
individual, enormalf plume structures can be clearly identified. Close to the poles these normal plumes blend together into a haze of emission, such that only
unusually bright plumes can be clearly identified.
EMovies suggest continuous outflow from plumes in a similar manner to the fan
loops of active regions. EIS spectroscopic measurements have demonstrated that
the fan loops in fact show redshifts (downflows), thus it is of great interest to measure the velocity shifts in the plumes and determine if the apparent outflows are real.
EPlumes do not necessarily (or rarely?) have bright footpoints, thus the standard magnetic reconnection model for plume formation is in question.
EBright points within coronal holes range from large (50h), fairly long-lived ones, to small (<20h) short-lived ones. Some coincide with plume footpoints, but many are spatially independent of plumes.
Figure 1 shows an image of the north coronal hole from 28 June taken in the 171 channel (Fe IX/X, ≈1 MK). This is a negative image, with black representing bright features, and it has been saturated to bring out weak emission. It is clear that the background in the coronal hole becomes significantly brighter (i.e., less emission) at the lower latitudes due to the line-of-sight effect and this enables individual plumes to be isolated. Examples are highlighted in the image. In addition the relation between plumes and bright points can be
made more easily.
While the AIA observations reveal the spatial and temporal details of coronal features to unprecedented levels, they raise questions that can only be answered through spectroscopic measurements. Namely:
EWhat are the velocities inside and outside of plumes? EIS has access to strong
lines of Fe VIII, IX and X that are ideal for measuring plume velocities.
EDo plumes without bright points visible in the 171 channel have bright points at cooler temperatures? EIS has access to lines formed down to 105 K, a region
inaccessible to AIA.
EHow are the plumes related to the spicular emission seen in He II 304? Can the
coronal blue-red asymmetries (De Pontieu et al. 2009) be identified in the EIS
lines? The line profiles of Fe VIII, IX and X can be measured to search for such
EWhat is the density and temperature of the plume emission, and how do they
compare with the dark, interplume lanes? Density diagnostics of Mg VIII and Fe
X are ideal for measuring plume densities, while EIS sees a complete range of
iron ions from Fe VII to Fe XVI that are ideal for identifying subtle temperature differences between regions.
Figure 1. AIA 171 image (negative) of the north coronal hole from 28 June.
The limited field-of-view of EIS, coupled with the low data-rate afforded by Hinode makes observing specific structures in a coronal hole difficult. The Hinode plans need to be prepared a long time in advance of the actual observation, but plumes are found to evolve significantly and even disappear on timescales of ~ 1 day. Therefore choosing a specific structure in advance is unlikely to be successful. Inspection of AIA movies suggests that some good luck is required to capture an isolated plume, and the ideal way to maximize chances is to observe continuously for a long period at a particular pointing,
and with as large a field-of-view as possible.
The EIS observation requested here covers 180hx512h in one hour, but to maintain continuous coverage for a complete 24 hour period with sufficient spectral coverage is not possible with the standard 15% data recorder allocation for the instrument.
For this reason we request that the standard Hinode data recorder allocations are requested to be changed to 50% for EIS, 15% for XRT and 35% for SOT. To minimize the impact on the regular Hinode observing campaigns it is requested that this change only occur for one OP period (be it 2 day or 3 day), and the present HOP would run for the complete duration of the OP. The reduced data recorder allocation for SOT provides significant constraints. Since HMI will obtain high cadence magnetograms, the key aim will be to obtain the highest spatial resolution magnetograms at the largest possible field-of-view. In addition Ca II images provide valuable information not provided by SDO. A cadence of 30-60 minutes should allow SOT to maintain coverage for the complete OP period.
A secondary aim of the HOP is to serendipitously make snapshot observations of some of the dynamic phenomena occurring in coronal holes. Jets are one such example, but the AIA data also reveal many short-lived (minutes), small (few arcseconds) brightenings that do not necessarily result in jets. By capturing these with EIS, instantaneous density and velocity measurements can be made that will complement the high cadence images of AIA. Similarly, the SOT data will yield high spatial resolution magnetic field snapshots that will complement HMI.
This proposed observation is much more efficient than if the EIS observations were performed with the normal data recorder allocations over several days, or sporadically over several months. In addition the window for observing the coronal hole is quite small since low latitude regions need to be observed and these regions must be close (within 2 days) of the central meridian.
Finally it is worth adding that the proposed observation will yield a very important archival data-set that will be of great value to other researchers in the future. Hinode rarely devotes continuous, multi-day periods to a single coronal hole pointing, while, since the failure of the X-band antenna, the EIS data rate has been too low to provide continuous observations.