Jonathan Cirtain

Evidence for Alfvén Waves in Solar X-ray Jets

Coronal magnetic fields are dynamic, and field lines may misalign, reassemble, and release energy by means of magnetic reconnection. Giant releases may generate solar flares and coronal mass ejections and, on a smaller scale, produce x-ray jets. Hinode observations of polar coronal holes reveal that x-ray jets have two distinct velocities: one near the Alfvén speed (∼800 kilometers per second) and another near the sound speed (200 kilometers per second). Many more jets were seen than have been reported previously; we detected an average of 10 events per hour up to these speeds, whereas previous observations documented only a handful per day with lower average speeds of 200 kilometers per second. The x-ray jets are about 2 × 103 to 2 × 104 kilometers wide and 1 × 105 kilometers long and last from 100 to 2500 seconds. The large number of events, coupled with the high velocities of the apparent outflows, indicates that the jets may contribute to the high-speed solar wind.

Energy release in the solar corona from spatially resolved magnetic braids

It is now apparent that there are at least two heating mechanisms in the Sun’s outer atmosphere, or corona. Wave heating may be the prevalent mechanism in quiet solar periods and may contribute to heating the corona to 1,500,000 K (refs 1, 2, 3). The active corona needs additional heating to reach 2,000,000–4,000,000 K; this heat has been theoretically proposed to come from the reconnection and unravelling of magnetic ‘braids’. Evidence favouring that process has been inferred, but has not been generally accepted because observations are sparse and, in general, the braided magnetic strands that are thought to have an angular width of about 0.2 arc seconds have not been resolved. Fine-scale braiding has been seen in the chromosphere but not, until now, in the corona. Here we report observations, at a resolution of 0.2 arc seconds, of magnetic braids in a coronal active region that are reconnecting, relaxing and dissipating sufficient energy to heat the structures to about 4,000,000 K. Although our 5-minute observations cannot unambiguously identify the field reconnection and subsequent relaxation as the dominant heating mechanism throughout active regions, the energy available from the observed field relaxation in our example is ample for the observed heating.

Observations of Separator Reconnection to an Emerging Active Region

Extreme-ultraviolet (EUV) observations of an emerging active region are used to study separator reconnection in the corona. We identify each EUV loop connecting the emerging polarity to a nearby existing active region over the 41 hr period beginning at emergence onset. Their geometrical resemblance to post-reconnection field lines from a magnetic model of the active region pair implicates separator reconnection in their production. While some reconnection is evident within 7 hr of emergence onset, the most intense period occurs after a 1 day delay. The sum of cross sections of all observed loops accounts for only one-fifth of the transferred magnetic flux predicted by the model. We suggest that the remaining loops remain at temperatures too high, or at densities too low, to be detected in our EUV data. The most intense reconnection requires as much as 109 V along the coronal separator; however, the observed loops suggests that the flux is transferred as discrete bundles of ~4 × 1018 Mx each. The reconnection appears to directly dissipate only a small fraction of the energy released, while the rest is dissipated within the post-reconnection flux over the ensuing 6 or more hours the loops remain visible. The net energy released, and ultimately dissipated, is consistent with the amount that could be stored magnetically during the 24 hr delay between emergence and reconnection.

Slipping Magnetic Reconnection in Coronal Loops

Magnetic reconnection of solar coronal loops is the main process that causes solar flares and possibly coronal heating. In the standard model, magnetic field lines break and reconnect instantaneously at places where the field mapping is discontinuous. However, another mode may operate where the magnetic field mapping is continuous but shows steep gradients: The field lines may slip across each other. Soft x-ray observations of fast bidirectional motions of coronal loops, observed by the Hinode spacecraft, support the existence of this slipping magnetic reconnection regime in the Suns corona. This basic process should be considered when interpreting reconnection, both on the Sun and in laboratory-based plasma experiments.

The Inadequacy of Temperature Measurements in the Solar Corona through Narrowband Filter and Line Ratios

We analyze the determination of coronal line-of-sight temperatures with the technique of narrowband filter ratios that is currently employed for data obtained with the Transition Region and Coronal Explorer and the EUV Imaging Telescope on board the Solar and Heliospheric Observatory. We demonstrate that the simple fact that the observed differential emission measure curves in coronal loops have a broad plateau everywhere along the length of the loop leads to the finding of isothermal loops with different temperatures for each pair of filters. We show that none of the temperatures thus obtained correctly describe the state of the loop plasma, which instead must be characterized by the full differential emission measure per pixel. We conclude that the recent discovery of a new class of isothermal loops is probably a mere artifact of the narrowband filter ratio method and show that the shift in the location of the plateau in the differential emission measure along the loop indicates significant heating near the loop tops.

Deciphering solar magnetic activity. I. On the relationship between the sunspot cycle and the evolution of small magnetic features

Sunspots are a canonical marker of the Suns internal magnetic field which flips polarity every ~22 yr. The principal variation of sunspots, an ~11 yr variation, modulates the amount of the magnetic field that pierces the solar surface and drives significant variations in our stars radiative, particulate, and eruptive output over that period. This paper presents observations from the Solar and Heliospheric Observatory and Solar Dynamics Observatory indicating that the 11 yr sunspot variation is intrinsically tied to the spatio-temporal overlap of the activity bands belonging to the 22 yr magnetic activity cycle. Using a systematic analysis of ubiquitous coronal brightpoints and the magnetic scale on which they appear to form, we show that the landmarks of sunspot cycle 23 can be explained by considering the evolution and interaction of the overlapping activity bands of the longer-scale variability.

DETECTING NANOFLARE HEATING EVENTS IN SUBARCSECOND INTER-MOSS LOOPS USING Hi-C

The High-resolution Coronal Imager (Hi-C) flew aboard a NASA sounding rocket on 2012 July 11 and captured roughly 345 s of high-spatial and temporal resolution images of the solar corona in a narrowband 193 A channel. In this paper, we analyze a set of rapidly evolving loops that appear in an inter-moss region. We select six loops that both appear in and fade out of the Hi-C images during the short flight. From the Hi-C data, we determine the size and lifetimes of the loops and characterize whether these loops appear simultaneously along their length or first appear at one footpoint before appearing at the other. Using co-aligned, co-temporal data from multiple channels of the Atmospheric Imaging Assembly on the Solar Dynamics Observatory, we determine the temperature and density of the loops. We find the loops consist of cool (~105 K), dense (~1010 cm–3) plasma. Their required thermal energy and their observed evolution suggest they result from impulsive heating similar in magnitude to nanoflares. Comparisons with advanced numerical simulations indicate that such dense, cold and short-lived loops are a natural consequence of impulsive magnetic energy release by reconnection of braided magnetic field at low heights in the solar atmosphere.

SOLAR X-RAY JETS, TYPE-II SPICULES, GRANULE-SIZE EMERGING BIPOLES, AND THE GENESIS OF THE HELIOSPHERE

From Hinode observations of solar X-ray jets, Type-II spicules, and granule-size emerging bipolar magnetic fields in quiet regions and coronal holes, we advocate a scenario for powering coronal heating and the solar wind. In this scenario, Type-II spicules and Alfv?n waves are generated by the granule-size emerging bipoles (EBs) in the manner of the generation of X-ray jets by larger magnetic bipoles. From observations and this scenario, we estimate that Type-II spicules and their co-generated Alfv?n waves carry into the corona an area-average flux of mechanical energy of ~7 ? 105?erg?cm?2?s?1. This is enough to power the corona and solar wind in quiet regions and coronal holes, and therefore indicates that the granule-size EBs are the main engines that generate and sustain the entire heliosphere.

Active region loops: Temperature measurements as a function of time from joint TRACE and SOHO CDS observations

In this paper, we aim to quantitatively investigate the structure and time variation of quiescent active region loop structures. We coordinated a joint program of observations (JOP 146) using TRACE, to obtain high-cadence EUV images, and SOHO CDS, to obtain spectroscopic data. Loop intensities are used to determine temperature as a function of time for a single loop, taking full account of the background emission. In many locations, the emission measure loci are consistent with an isothermal structure. However, the results indicate significant changes in the loop temperature (between 1 and 2 MK) over the 6 hr observing period. It is possible that the loop structures are composed of multiple, independently heated strands with sizes less than the resolution of the imager and spectrometer.

ARE CORONAL LOOPS ISOTHERMAL OR MULTITHERMAL

Surprisingly few solar coronal loops have been observed simultaneously with TRACE and SOHO/Coronal Diagnostics Spectrometer (CDS), and even fewer analyses of these loops have been conducted and published. The SOHO Joint Observing Program 146 was designed in part to provide the simultaneous observations required for in-depth temperature analysis of active region loops and determine whether these loops are isothermal or multithermal. The data analyzed in this paper were taken on 2003 January 17 of AR 10250. We used TRACE filter ratios, emission measure loci, and two methods of differential emission measure analysis to examine the temperature structure of three different loops. TRACE and CDS observations agree that Loop 1 is isothermal with log T = 5.85, both along the line of sight as well as along the length of the loop leg that is visible in the CDS field of view. Loop 2 is hotter than Loop 1. It is multithermal along the line of sight, with significant emission between 6.2 < log T< 6.4, but the loop apex region is out of the CDS field of view so it is not possible to determine the temperature distribution as a function of the loop height. Loop 3 also appears to be multithermal, but a blended loop that is just barely resolved with CDS may be adding cool emission to the Loop 3 intensities and complicating our results. So, are coronal loops isothermal or multithermal? The answer appears to be yes.