[Loops] loops and thermal nonequilibrium

[Leon Golub]

Dear Tongjiang,

I see that this subject is going to be debated for many years to come, 
which is good. We'll have to discuss this in person some time, where we 
show each other the observations and both be looking at the same thing. 
In particular, it is exactly in those fan-like features that we see the 
clearest flows, and indeed there is some question about the temperature.
We need simultaneous observations, in the same spectral lines, with EIS 
and AIA. These should be available in a few months, once SDO launches. 
Since TRACE sees upflows at both ends of the large loop systems, it 
seems likely that there are interlaced loops (or threads) and that the 
flows go up warm and are cooler coming down. With low-resolution (~1") 
imaging, it would seem as if the blue shifts and red shifts are on top 
of each other, but this is an illusion cause by failure to resolve the 
different structures.

Leon

Tongjiang Wang wrote:
> Dear Leon,
> 
> I just would like to remind you that the regions showing tens of km/s 
> upflows seen in EIS coronal lines are typically for weak or dark fan 
> structure in TRACE and XRT, while the propagating wave features are 
> often seen in bright fan-like loops (like those shown in my paper,
> where the brighter part of fan loops show RED-SHFIT!). Indeed there are
> other EIS observations show bright fan-like loops show redshift while
> not blue shifts. Moreover, as you mentioned the steady flow is not 
> possibly seen in imaging observations, that means that the moving brobs 
> features correspond to episodic jets or pulsive flows, thus Doppler 
> shift fluctuations of tens of km/s amplitudes are expected for a upflow 
> of 100-120 km/s for fan-like loops with a typical inclination of 50-60 
> deg (see Marsh et al. 2009 who derived the inclination from STEREO/EUVI 
> obs), but my study did not
> show this is the case. Finally, in the dark region showing tens of km/s
> EIS blue shifts, the waves may co-exist with upflows, therefore, the 
> upwards moving blobs are not necessary to be the upflows. I'll try to 
> look for more examples to support my view. Thank you for your discussions.
> 
> Best Regards
> 
> Tongjiang
> 
> On Mon, 21 Dec 2009, Leon Golub wrote:
> 
>> Dear Tongjiang,
>>
>> This is exactly the "argument" I've been having for the past ten 
>> years, starting with Nakariakov in 1999. I've been waiting for the 
>> Doppler data since then, because where I see flows, other people see 
>> waves. I do think that the EIS results are now showing higher 
>> velocities of some tens of km/sec, and in TRACE the flows do start out 
>> slow at the footpoints, so there's no inconsistency with what you're 
>> saying about the speeds. There is a difference in that EIS sees the 
>> upflows at somewhat higher temperatures than we do with TRACE, and XRT 
>> sees them too, meaning higher T. I'm hoping that AIA will help sort 
>> this out.
>>
>> What I can tell you is that when we see waves in the corona, they are 
>> quite clearly periodic and there's no doubt about it. Just look at any 
>> movie (with high time cadence) taken above sunspots: it looks like a 
>> loudspeaker pulsing away. These fans are different, and if you look 
>> closely at your time-distance plots, you'll see the lower portions 
>> curving upward, as the flows accelerate on their way up.
>>
>> Leon
>>