Relativistic Jet Simulations of the Weibel Instability in the Slab Model to Cylindrical Jets with Helical Magnetic Fields

Round 1

Reviewer 1 Report

Major comments:

The authors use low values of magnetization for their simulations when a number of RMHD simulations, analytical work, and magnetic reconnection studies agree that σ>1 is most likely relevant for astrophysical jets. As the authors themselves state Weibel instability appears in low magnetized plasmas which initially justifies the use of low values for the magnetization, but later in the paper perform simulations including different kinds of instabilities. Given the literature results: how relevant is the Weibel instability for jets? How does the Weibel instability compare to the other instabilities considered in their simulations of plasma with higher magnetization? Some discussion of the above points is in my opinion necessary, otherwise it could be misleading to the reader.

It will also be good to include more additional details/clarifications on the simulations such as, is the same range of magnetization values used throughout the paper? what is the ratio of electrons to protons in the simulations? Does that vary in the different simulations?
   


Minor comments:

Paragraph 1: the authors should include X-ray binaries since they too create relativistic jets.

line 22: time-variabilities --> time-variability.

line 22: polarity--> I suspect the authors mean "polarization".

line 24: universe --> Universe.

line 25: "propagation in the interstellar medium" is not a plasma phenomenon.

line 38: It might be better for the references to be at the end of the sentence.

line 40,41,42: The sentence is a bit confusing, I suggest the authors rephrase.

line 95: "moving jet particles" is written twice.

line 102,104: There are references missing.

line 122,123: The extra paragraph is not necessary.

line 236: "...of relativistic jets is intrinsically cylindrical...", the structure of the jets is a field of very active research with no widely accepted consensus. This sentence is somewhat misleading. I would suggest rephrasing it.

line 257: There is a reference missing.

line 263,264,265: This paragraph is redundant. If the authors find it necessary, they can extend the last paragraph in the introduction.

line 272: The references are not generated correctly.

line 337: Initial --> initial.

line 386: refecting --> reflecting.


Author contributions: Some authors are not listed, some have repeated entries, and there are some typos. Also, some authors are stated as responsible for comparison to observations and RMHD simulations, yet there is no such comparison the text.

Throughout the text there are "the" missing, new paragraphs that are not necessary, and some sentences that are not wrong, but sound non-native English. Since there are native English speakers among the authors it is worth taking a second look at the whole text to improve readability.

Author Response

see the attached file

Author Response File: Author Response.docx

Reviewer 2 Report

Overall the paper is a nice summary of the group's studies towards global simulations of the jet and the particle acceleration processes therein. The paper considers Weibel instability, shear layer, and magnetic reconnection processes in jets, and summarizes the group's research on the particle acceleration.

However, I feel that the paper is too optimistic about global PIC simulations, which should have several major issues that currently cannot be resolved. Therefore, some key conclusions that the paper made actually cannot stand. I strongly suggest that the authors should consider these issues and/or reasonably discuss them in the paper.

The paper studies how the global jet dynamics may be affected by instabilities. In radio observation, we often see moving and standing knots near the central engine, and also bending trajectories, etc. see papers by the MOJAVE project or Boston University group. These features happen on scales of pc to tens of pc, which are often considered as "global jet evolution". On the other hand, the particle kinetic scales, for example, the gyroradius of electrons, are on the scale of <~10^10 cm (skin depth etc are of more or less the same order). This means a scale difference of at least 8 orders! Therefore, even the paper uses simulation setup as in a global jet simulation, such as Mizuno's kink paper, the results should only apply to microscopic scales but certainly not the "global jet dynamics".

Of course, there are several works claim that the PIC results are self-similar and can be applied to large scales, such as studies by Lorenzo Sironi's group and Fan Guo's group, the radiative cooling and adiabatic expansion become very important, in particular if the authors want to claim some radiation signatures as implied at the end of the paper. There are a couple of papers by Petropoulou et al. and Zhang et al. discussing about these effects on the observed radiation and polarization signatures. While even these works have the above scale difference problem, the authors' simulations do not even fully consider these effects. Therefore, I do not believe that their simulations can result in any reliable predictions of radiation signatures either.

That said, I do consider the paper very interesting, because in any case a self-consistent global PIC simulation is the direction that the community is pursuing, so it is very nice to see some first trials. Therefore, I suggest that the authors should just recognize these issues in the paper, and if possible, discuss how to overcome these problems in future works.

Otherwise, there are a few minor issues in the paper.

There are a few quantities that are not defined when they first appear. For example, magnetization factor \sigma on page 4.

There are a few missing references or typos, for example, a ? mark on page 8 and nishikawa14b on page 9.

Author Response

see the attached file

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The new draft addresses my major concern appropriately. Therefore, I think the draft is in very good shape.

One minor comment that the authors may want to think about.

In the discussion part the authors mentioned a radiative transfer code developed by MacDonald and Marscher. Frankly I do not see a direct link to the other parts of the paper draft, as the paper does not put much effort to explain observations or try to discuss any radiation signatures.

I pointed out this lack of connection to observation in my previous review report as well. Apparently the authors still want to include this part, then it makes more sense to elaborate a little bit on it. Anyway, not a big issue though.

Author Response

see the attached file

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

The authors addressed my concerns, and the draft looks all good now.