3.1 General properties

Although coronal loops are often well defined and studied in the EUV band, detected by many space mission spectrometers like those on board SoHO and Hinode, and by high resolution imagers such as NIXT and TRACE, the bulk of coronal loops is visible in the X-ray band (Figure 1View Image). Also, the peak of the coronal emission measure of active regions – where the loops are brightest – is above 2 MK, which is best observed in X-rays (e.g., Peres et al., 2000Jump To The Next Citation PointReale et al., 2009aJump To The Next Citation Point).
View Image

Figure 1: Images of the same active region, taken in the EUV band with TRACE (top) and in the X-ray band with Hinode/XRT (bottom), on 14 November 2006. The X-ray image shows more clearly that the active region is densely populated with coronal loops.

Coronal loops are characterized by an arch-like shape that recalls typical magnetic field topology. This shape is replicated over a wide range of dimensions. Referring, for the moment, to the soft X-ray band, the main properties of coronal loops are listed in Table 1. The length of coronal loops spans over at least 4 orders of magnitude: bright points (∼ 108 cm), small Active Region (AR) loops (∼ 109 cm), AR loops (∼ 1010 cm), giant arches (∼ 1011 cm) (Figure 2View Image). As already mentioned, the loops owe their high luminosity and variety to their nature of magnetic flux tubes where the plasma is confined and isolated from the surroundings. Magnetized fully-ionized plasma conducts thermal energy mostly along the magnetic field lines. Due to the high thermal insulation, coronal loops can have different temperatures, from ∼ 105 K (cool loops), to a few 106 (X-ray loops), up to a few 107 K (flaring loops). A density of the confined plasma below 107 – 108 cm–3 can be difficult to detect, while more typical values of bright loops are 109 – 1010 cm–3 in quiescent and active regions loops. Flaring loops can be easily a factor 10 denser. The corresponding plasma pressure can typically vary between 10–3 and 10 dyne cm–2 for non-flaring loops, corresponding to confining magnetic fields of the order of 0.1 – 10 G. One characterizing feature of coronal loops is that typically their cross-section is constant along their length above the transition region, at variance from the topology of potential magnetic fields. There is evidence that the cross-section varies across the transition region, as documented in Gabriel (1976Jump To The Next Citation Point).

Table 1: Typical X-ray coronal loop parameters
Type Length Temperature Density Pressure
  [109 cm] [MK] [109 cm–3 ] [dyne cm–2]
Bright points 0.1 – 1 2 5 3
Active region 1 – 10 3 1 – 10 1 – 10
Giant arches 10 – 100 1 – 2 0.1 – 1 0.1
Flaring loops 1–10 > 10 > 50 > 100

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Figure 2: The X-ray corona contains loops with different spatial scales, e.g., bright points (BP), active region loops (AR), large scale structures (LSS). Credit: Yohkoh mission, ISAS, Japan.

3.1.1 Classification

Myriads of loops populate the solar corona and constitute statistical ensembles. Attempts to define and classify coronal loops were never easy, and no finally established result exists to-date. Early attempts were based on morphological criteria, i.e., bright points, active region loops, and large scale structures Vaiana et al. (1973Jump To The Next Citation Point), largely observed with instruments in the X-ray band. In addition to such classification, more recently, the observation of loops in different spectral bands and the suspicion that the difference lies not only in the band, but also in intrinsic properties, have stimulated another classification based on the temperature regime, i.e., cool, warm, and hot loops (Table 2). Cool loops are generally detected in UV lines at temperatures between 105 and 106 K. They were first addressed by Foukal (1976Jump To The Next Citation Point) and later explored more with SoHO observations (Brekke et al., 1997Jump To The Next Citation Point). Warm loops are well observed by EUV imagers such as SoHO/EIT and TRACE, and confine plasma at temperature around 1 – 1.5 MK Lenz et al. (1999Jump To The Next Citation Point). Hot loops are those typically observed in the X-ray band and hot UV lines (e.g., Fe xvi), with temperatures around or above 2 MK (Table 1). These are the coronal loops already identified, for instance, in the early rocket missions Vaiana et al. (1973Jump To The Next Citation Point). This distinction is not only due to observation with different instruments and in different bands, but there are hints that it may be more substantial and physical, i.e., there may be two or more classes of loops that may be governed by different regimes of physical processes. For instance, the temperature along warm loops appears to be distributed uniformly and the density to be higher than that predicted by equilibrium conditions. Does this make such loops intrinsically different from hot loops, or is it just the signature that warm loops are a transient conditions of hot loops?

Table 2: Thermal coronal loop classification
Type Temperature [MK]
Cool 0.1 – 1
Warm 1 – 1.5
Hot ≥ 2

A real progress in the insight into coronal loops is expected from the study of large samples of loops or of loop populations. Systematic studies of coronal loops suffer from the problem of the sample selection and loop identification, because, for instance, loops in active regions overlap along the line sight. Attempts of systematic studies have been performed in the past on Yohkoh and TRACE data (e.g., Porter and Klimchuk, 1995Jump To The Next Citation PointAschwanden et al., 2000Jump To The Next Citation Point). A large number of loops were analyzed and it was possible to obtain meaningful statistics. However, it is difficult to generalize the results because of limited samples and/or selection effects, e.g., best observed loops, specific instrument. One basic problem for statistical studies of coronal loops is that it is very difficult to define an objective criterion for loop identification. In fact, loops are rarely isolated; they coexist with other loops which intersect or even overlap along the line of sight. This is especially true in active regions where most of the loops are found. In order to make a real progress along this line, we should obtain loop samples and populations selected on totally objective and unbiased criteria, which is difficult due to the problems outlined above. Some steps are coming in this direction and we will see results in the future.

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